Unfortunately, sometimes, insider cheating can happen on online poker websites. The biggest example of that is if an employee of the website knows of a means of seeing extra information about games. The employee could then play games themselves and win, easily. Unfortunately, the site managers may have no idea whatsoever that that is happening until long after it is too late.
In October of 2007 a major case of insider cheating came to light. One of the major poker websites, Absolute Poker, admitted, publicly, that an employee had cheated the system. The employee had been playing at the high stakes tables using a cheat that allowed him to see cards in his opponents' hands that were supposed to be hidden. Luckily, through the efforts of some of the players, the person was caught. The players used their recorded hand histories to prove that the cheater was using extra information.
Another major insider cheating scandal went public the following year. On May 29, 2008, UltimateBet, another major poker website, announced a case of insider cheating. An employee of UltimateBet was using a back door in the software. Using that, the employee had been able to see his opponents' hole cards, thus giving him an unfair advantage. As a result, sanctions against UltimateBet were announced by the Kahnawake Gaming Commission.