Pokémon Generation I Bugs, Limitations & Restrictions

As Generation I was the first iteration of Pokémon video games, it was natural for there to be ideas missing that would only be realised in later installments on future platforms. Additionally, due to the methods used to code the game and the length of time that the video game has been available to analyze, there has been an extensive list of unintentional bugs, limitations and game behaviour uncovered. Unless otherwise stated, all of the below applies to the international releases of Pokémon Red Version, Pokémon Blue Version and Pokémon Yellow Version: Special Pikachu Edition.

Move Bugs

 * PP are not tracked on enemy Pokémon. During a battle, your opponent will never run out of PP for their moves.
 * Critical Hits were based on the Pokémon's Speed stat, but they occurred approximately four times more than they were supposed to, causing fast Pokémon to perform a comparatively large number of Critical Hits.
 * The moves Focus Energy and Dire Hit actually cut the Critical Hit chance of the Pokémon that use it by 75%, rather than multiplying the chance of Critical Hits by four.
 * Agility not only completely eliminates the Speed penalty caused by the Paralysis status (i.e. restores your Speed stat to normal), but it then doubles the Speed stat as per its effect immediately afterwards.
 * Blizzard, a move with 5 PP, has an accuracy rate of ~90%, which is very large considering the power of the move
 * If your Pokémon was defrosted during a turn in which you had not yet attacked, you will automatically use the last move you used during that battle. If you did not attack at all that battle, you will use a glitched attack.
 * Whirlwind and Roar do nothing in Link Battles.
 * You can evolve a Pokémon that would normally require a Stone. They would need to level up in a Trainer Battle with the finishing Pokémon's internal number matching the internal number of required Stone.
 * Mimic randomly mimics an enemy move, which can cause duplicate moves on that Pokémon's moveset.
 * The only Dragon-type move is Dragon Rage, which always deals a fixed amount of 40 damage.
 * Moves like Night Shade, Counter and Bide ignore type immunities.
 * Ghost-type Pokémon are immune to the Struggle move because it's considered a Normal-type move (rather than being programmed as a Typeless move like it is in later generations).
 * If you use a PP Up on a move, you cannot use Ethers or Elixers on it.
 * Psychic-type Pokémon are immune to Ghost-type moves, meaning that the only Pokémon capable of resisting Psychic-type moves are other Psychic-Type Pokémon.
 * The AI would prioritize Super Effective moves, even if they were non-damaging.
 * If you switch out your Pokémon during a Trainer battle, the AI picks their move after the switch out, not before, which means the AI has the opportunity to pick a move that takes advantage of a type disadvantage for the Pokémon switched in.
 * A Pokémon waking up from the Sleep status requires a whole turn; no attack can be made by a Pokémon the turn it wakes up.
 * Attacking moves that have a chance to inflict the Paralyze Status effect don't apply that status to Pokémon of the same type as the move being used; Lick cannot Paralyze Ghost-Type Pokémon, Thunderbolt cannot paralyze Electric-type Pokémon, and Body Slam cannot paralyze Normal-type Pokémon.
 * If you use two or more Pokémon in a single battle, the total experience that Exp. All grants significantly drops.
 * If a Pokémon's stats change outside of a Link Battle, any and all stat bonuses you get from Gym Badges are reapplied (around ~12.5%) to all stats.
 * If one or more stats were modified when the non-turn player's Pokémon is Paralyzed or Burned, the Speed or Attack drop from that status would be reapplied again. Using a move like Swords Dance while the opponent's Pokémon is Paralyzed causes the opponent's Speed to be divided by four; repeatedly doing this can cause the relevant stat to drop all the way down to one point.
 * If you level up a Pokémon too fast by gaining multiple levels, the Pokémon will not learn moves that would have been learned at the levels that were skipped.
 * If you level up a Caterpie or Weedle into Metapod or Kakuna respectively, the newly-evolved Pokémon does not learn the move Harden. Meanwhile, Metapods and Kakunas caught in the wild only know Harden.
 * If during a link battle, you switched out Mirror Moved trapping move like Fire Spin, the move would hit for one player, and miss for the other, instantly desynchronizing the link battle.
 * If a Pokémon attacks with a damaging move, if either the Attack stat of that Pokémon or the Defense stat of the opponent's monster is higher than 255, the game divides the stat by four to ensure the result can be recorded in a single byte of data. If the other stat was 3 or lower, the game would first lower it to zero, and then try to divide it by four. This would cause the game to freeze.
 * If Reflect or Light Screen is used with a Defense stat of 512 or higher, the Defense stat underflows into very low numbers. If the Defense stat was exactly 512 or 513, the game sets it to zero, and then freezes.
 * Psywave causes at least 1 HP damage when used by the player, but is capable of causing 0 HP of damage when used by the opponent. Because of this, it's possible to desynchronize link battles with this move.

Bide

 * If you use Bide, and the enemy uses only certain non-attacking moves, or switches out, it counts as if they has used whatever they hit you with the previous turn for Bide. It's even possible to accumulate damage for Bide from a previous battle if no new damage data exists.
 * Every move in the game, except Swift and Bide, has a base 1/256 chance of missing.
 * Swift and Bide will hit a Pokémon that would normally be invulnerable during the opening move of Fly or Dig.
 * If you use Dig or Fly, but miss the main attack due to Paralysis or Confusion, that Pokémon remains in the invulnerable state until either swapped out, or that same move is used again, and can only be hit with Swift or Bide, essentially making the Pokémon invulnerable.
 * If an enemy is KO'd while you're storing damage with Bide, most of the damage remains stored, while if the reverse is true (enemy uses Bide and your Pokémon is KO'd), all the accumulated damaged in Bide's damage counter is reset. This tactic will desynchronize link battles.

Counter

 * Counter can be used on One-Hit KO moves to instantly knock-out the opponent.
 * Counter can only be used on Normal or Fighting-type moves.
 * Counter can be used against fixed-damage moves like Seismic Toss or Dragon Rage.
 * Counter can also be used to bounce your own moves back to the opponent if the opposing Pokémon hasn't used any move after your last attack that caused damage (or one of a small handful of moves that don't interfere with the data of the damage from the last attack).
 * Counter can also be used with a previous use of the Counter move.

Haze

 * If a Pokémon is afflicted with the Frozen status, that Pokémon will never thaw out on its own. Apart from using items like Ice Heal of Full Heal, the only two ways to unfreeze a Pokémon is to use a Fire-type move on it, or have the opposing Pokémon use Haze, which would thaw out the Pokémon in addition to its normal effect.
 * Haze also cures the Sleep status.
 * If a Pokémon is cured of either Freeze or Sleep by Haze, that Pokémon is unable to use any attack until it leaves the field.

Hyper Beam

 * One-Hit KO moves like Solar Beam and Hyper Beam were also based on the Speed stat, making them largely useless due to the fact that mostly slower Pokémon learned moves like these.
 * If a Pokémon is knocked out with Hyper Beam, there is no missed turn to recharge.
 * If a Pokémon has to recharge from using Hyper Beam, and a move that causes the Sleep status (like Hypnosis or Sleep Powder) is used on it, the move completely ignores accuracy and always hits. If the Pokémon is afflicted with another status, like Paralysis or Poison, that status is removed when Sleep is applied.
 * If a Pokémon used Hyper Beam, but the opponent uses Wrap, Bind, Fire Spin or Clamp which then misses, rather than recharge, the Pokémon that just used Hyper Beam will use it again instead of recharging.

Jump Kick/High Jump Kick

 * Jump Kick and High Jump Kick only do 1 HP of crash damage to the Pokémon that uses those moves, rather than the 1/8 damage they are supposed to do.
 * If you have a substitute up, the crash damage from Jump Kick and High Jump Kick is dealt to your opponent (if they have a substitute up), or if they don't have a substitute up, no damage is dealt at all.

Leech Seed

 * If a Poison or Burn status causes a Pokémon to be knocked out, and Leech Seed is currently active, Leech Seed would still drain HP before the Pokémon was removed from the field, despite the target Pokémon having no HP left at this stage.
 * Leech Seed, Toxic and the Burn status use the same residual damage calculator; using these moves together will cause the damage to become stacked; Leech Seed will deal double the damage that Toxic deals, while Toxic will deal double the damage that Leech Seed deals at the end of each turn, causing the residual damage to multiply by a factor of two each time one of those effects resolves.
 * If the Pokémon had less HP than what Leech Seed would normally drain, the Pokémon using Leech Seed would still receive the amount of HP that Leech Seed would have normally drained, rather than the reduced amount that the target Pokémon had.

Rage

 * When Rage is used, the Pokémon will stop obeying its trainer, and will be unable to use any other moves; it will keep using Rage until it is KO'd or switched out.
 * Rage never loses any PP apart from the 1 PP expended for the original attack.
 * If, while using Rage, your Accuracy goes down or the enemy's Evasion goes up, your Accuracy will keep going down until it its Accuracy becomes 1-in-256.
 * If Rage ever misses, its Accuracy becomes 1-in-256.
 * While Disable correctly randomly disables an enemy move, it also increases the damage counter used by Rage.
 * If a Pokémon is put to Sleep, or is Frozen in the middle of using a multi-turn attack like Solar Beam, Thrash, or Rage, and the Pokémon in question is cured of its Sleep or Frozen status with Haze, that Pokémon cannot use any attack at all until it is swapped out.

Rest

 * If a Pokémon is afflicted with Toxic, and Rest is used, the Poison status is cured, but the residual damage counter isn't reset. This means that if Leech Seed or Toxic is used or the Burn status is reapplied, high amounts of damage can be caused by the counter starting from the previously-saved value of the residual damage counter.
 * While Rest cures the Paralysis or Burn status, it doesn't negate the stat penalties from the status being originally applied unless that Pokémon is switched out.
 * If your Pokémon's HP is 255 or 511, recovery moves like Recover and Rest will fail.
 * If a Pokémon is afflicted with Toxic, and Rest is used, the Poison status is cured, but the residual damage counter isn't reset. This means that if Leech Seed or Toxic is used or the Burn status is reapplied, high amounts of damage can be caused by the counter starting from the previously-saved value of the residual damage counter.

Substitute

 * Substitutes don't protect you from Status Effects, with the exception of Poison or Confusion.
 * KO'ing a substitute will cancel the secondary effect of most moves, including Hyper Beam's turn to recover, or the recoil from certain attacks. If a Substitute is KO'd by Explosion or Self-Destruct, the Pokémon using the move isn't KO'd themselves.
 * If you have a substitute up, the crash damage from Jump Kick and High Jump Kick is dealt to your opponent (if they have a substitute up), or if they don't have a substitute up, no damage is dealt at all.
 * If you check the status page for one of your Pokémon after the opponent has used Minimize or Substitute, the sprite displayed will be corrupted.
 * Counter will also work for moves used on a substitute.
 * Moves that drain HP, like Absorb, work on substitutes, when they were supposed to miss.
 * If you have a substitute up, and you hit yourself due to Confusion, the damage is dealt to your opponent, if they have a Substitute up, or if they don't have a Substitute up, no damage is dealt at all.
 * If you have exactly 25% of your HP remaining when you use the move Substitute, you will KO your Pokémon.

Swift

 * Every move in the game, except Swift and Bide, has a base 1/256 chance of missing.
 * Swift and Bide will hit a Pokémon that would normally be invulnerable during the opening move of Fly or Dig.
 * If you use Dig or Fly, but miss the main attack due to Paralysis or Confusion, that Pokémon remains in the invulnerable state until either swapped out, or that same move is used again, and can only be hit with Swift or Bide, essentially making the Pokémon invulnerable.

Swords Dance

 * Swords Dance not only completely eliminates the Attack penalty caused by the Burn status (i.e. restores your Attack stat to normal), but it then doubles the Attack stat as per its effect.
 * Critical Hits ignore stat-ups from both Pokémon rather than only the target. Using Swords Dance three times would make your Pokémon cause more damage than a Critical Hit; additionally, as might be observed during the early part of the game, if your opponent has Growl used on it repeatedly, and scores a Critical Hit, the hit does damage as if Growl was never used.

Toxic

 * Leech Seed, Toxic and the Burn status use the same residual damage calculator; using these moves together will cause the damage to become stacked; Leech Seed will deal double the damage that Toxic deals, while Toxic will deal double the damage that Leech Seed deals at the end of each turn, causing the residual damage to multiply by a factor of two each time one of those effects resolves.
 * If a Pokémon under the effect of Toxic is swapped out, and then swapped back in again, the Poison damage is reduced to normal Poison damage, rather than the increased Poison status damage that Toxic inflicts.
 * If a Pokémon is afflicted with Toxic, and Rest is used, the Poison status is cured, but the residual damage counter isn't reset. This means that if Leech Seed or Toxic is used or the Burn status is reapplied, high amounts of damage can be caused by the counter starting from the previously-saved value of the residual damage counter.

Transform

 * If you capture a Pokémon that has used Transform (even if a Spearow used Mirror Move, gained Transform, and then used Transform), the game automatically assumes you've caught a Ditto.
 * If you use a Ditto to transform into a Pokémon that has more than one attack, and then alter the position of moves within that moveset, you can cause Ditto to learn a glitched move (since the moves that are being moved don't exist).

Wrap/Bind/Clamp/Fire Spin

 * If you use the last of the PP for Wrap, Bind, Fire Spin or Clamp, but the opponent switches out the Pokémon, the PP will underflow to become 63, and will have had the effect of three PP Ups applied to the standard max amount of PP for that move.
 * The moves Bind, Wrap, Fire Spin and Clamp render the target Pokémon unable to use any moves for 2-5 turns. Switching out your Pokémon doesn't help, as the Pokémon that gets switched in is still counted as being trapped by the same move. If your Pokémon is afflicted by one of these moves, and your Pokémon is slower than the opponent, you need to wait until the opponent misses, or a different move is used.
 * If you decide to use an item while you think your Pokémon is still under the effect of Wrap, Bind, Fire Spin or Clamp, but the attack ended the previous turn, and the opposing Pokémon chooses to use a move that inflicts the Sleep status on your Pokémon, your Pokémon will never wake up on its own.
 * For multi-hit moves like Wrap, Bind, Fire Spin or Clamp; if the first hit lands a Critical Hit, they all land Critical Hits.

Game Behaviour
The Exp. All significantly decreases the total amount of Exp earned if two or more of your Pokémon participe in a battle.
 * You have to save every time you change Pokémon boxes.
 * The games will allow you to deposit all your Pokémon into a box, leaving you with fainted Pokémon. If you then move three spaces, you will black out.
 * Blaine could use Super Potions even when his active Pokémon was at max HP.
 * You can bypass the Rocket Hideout event if you use a Poké Ball on the ghost Marowak in Pokémon Tower.
 * Depending on the target Pokémon's health and status, Great Balls could have a greater catch rate than Ultra Balls.
 * If you catch a Pokémon, your own Pokémon don't gain any experience.
 * If you hit, or get hit with a Super Effective move, the message and sound played only reflected whether or not the move was Super Effective against the type of the two that has its type matchup coded first.
 * You can evolve Pokémon that would normally require an evolutionary stone; they need to level up during a trainer battle, and by finishing the battle with a Pokémon that has an internal number matching the internal number of the required stone:
 * For Pokémon that evolve with a Moon Stone the battle needs to finish against an Exeggutor.
 * For Pokémon that evolve with a Fire Stone the battle needs to finish against MissingNo.
 * For Pokémon that evolve with a Thunder Stone the battle needs to finish against a Growlithe.
 * For Pokémon that evolve with a Water Stone the battle needs to finish against an Onix.

Limitations

 * There is no Move Deleter NPC. Because of this, once an HM move (like Cut, Fly, Surf, Strength and Flash) has been taught to a Pokémon, it cannot be removed, either with a TM, or a move the Pokémon learns naturally.
 * Charizard cannot learn Fly in Pokémon Red Version or Pokémon Blue Version, despite having the Flying type as its second type. This was fixed in Pokémon Yellow Version: Special Pikachu Edition.
 * The Special stat was referenced for both Attack and Defense, which meant that a Pokémon with a high Special stat would simultaneously be good for attacking with Special attacks and good for defending against Special attacks. This was rectified in Generation II, when the Special stat was split into Special Attack and Special Defense.
 * Items that can be held by Pokémon were introduced in Generation II. If compatible Pokémon are transferred from either Pokémon Gold Version, Pokémon Silver Version or Pokémon Crystal Version, it is recommended that items be removed prior to the transfer to either Pokémon Red Version, Pokémon Blue Version, or Pokémon Yellow Version: Special Pikachu Edition.