Rules Are for Humans

In a computer adaptation of an existing game, the AI may have the ability to pull off moves which are against the rules of the game.

Note that this does not include simple extra rules. For example, a Chess variant in which the queens can move as knights isn't cheating if applied consistently. However, if the AI's queen can move as a knight but the player's queen can't, then that's cheating.

Compare My Rules Are Not Your Rules and The Computer Is a Cheating Bastard.

Examples of Rules Are for Humans include:

A certain chess program, when it was close to losing, would actually flash the message "The [piece] has escaped!" and one of the captured pieces would reappear on the board. Obviously, only the computer's pieces ever 'escaped'. One hopes that this isn't how Deep Blue beat Garry Kasparov (judging by the fact that Deep Blue wasn't smashed to pieces under mysterious circumstances shortly afterward, it probably wasn't).
Subverted in other chess games when users see the computer use "en passant" (an oddball move that allows one pawn to capture another in a weird way, under very specific circumstances) and think it's cheating. In fact, "en passant" is entirely legal and you can do it too, if you know how it works. YouTube is full of videos by idiots who think the computer is cheating when, in fact, they themselves just don't know how to play chess.
- Of course there is the issue of games with shoddy user interfaces that won't let the player enter the moves, but do allow the computer to do it.
The National Lampoon chess game (yes, there was such a thing) cheated constantly, though this was part of the point.
The final boss in Yu-Gi-Oh Reshef of Destruction starts with 100,000 life points, when the rules say players start with 8,000. He can also heal himself without using a card.
- The final boss of the Microprose Magic: The Gathering PC game also has an absurd amount of health, in addition to starting with several permanents in play. However, by the time you fight him, you've already foiled his plans; win or lose, you win, and the amount of damage you rack up simply determines how long he's banished from your world. Additionally, his five-color deck is quite impractical, so it's very possible to run down his health as he tries to get the proper types of mana to cast his spells.
Yu-Gi-Oh computer games are fond of this all around. Yu-Gi-Oh Online Duel Evolution provides you with NPCs to duel as well as other humans. The game imposes the current ban/restricted lists of Yu-Gi-Oh on your deck, meaning there's certain cards you can't use even though you may own them. The NPCs are not under these restrictions, leaving you saying, "Boy, I wish I could play Pot of Greed/Snatch Steal/Ring of Destruction."
- Or, for games like the Tag Force series, "Boy, I wish I could play three copies of a card that's limited to one".
The final boss of the first chapter of Digimon: Digital Card Battle cheats as a plot device.
In the Dragonball Z CCG video game, Cell gets the extra benefits of cards that only grant those benefits if you declare a Favorite Fighting Style Advantage despite not qualifying.
- This is somewhat justified in-story by the fact that he's a DNA composite of all the heroes.
A computer version of Avalon Hill's board game Diplomacy allows fleets to support attacks into adjacent inland provinces or coastal provinces that share only a land border, though this is more likely a programming glitch than deliberate cheating.
Inverted in Neverwinter Nights 2, with the Frenzied Berserker. In the Dungeons & Dragons rules, that class gets an ability called "Deathless Frenzy" which means the berserker can not die as long as his frenzy lasts. Players in the video game are mysteriously missing this power, but a boss enemy does have it—meaning that in this case it's the computer who follows the rules.
There was once an electronic Tic Tac Toe game in the Sydney Powerhouse Museum, which was literally impossible to win, thanks to this trope. The computer's flawed programming meant that it could override a box you had put your nought in with one of its own crosses, meaning that the computer would always win in three or four moves, no matter what you did.
Baldur's Gate is based on the 2nd edition AD&D rules of Dungeons & Dragons and generally follows a Pragmatic Adaptation of the rules. Enemy mages, however, break the system. They're able to put spells into Spell Triggers and Contingencies that are not legal for player characters in tabletop or the game, they're able to have multiple Spell Triggers and Contingencies (which is also illegal) and finally they have the ability to use an ability called the 'tattoo of power' to grant them an extra layer of instant defence, which does not exist in Pen-And-Paper and is inaccessible to the party.
AI players in the 2010 Blood Bowl adaptation randomly acquire traits, which can lead to AI players with the grab + frenzy combo. This combo is outright stated to be illegal in the Blood Bowl rulebook, and in both traits' descriptions in-game.
In World of Warcraft, the Chess event in the Karazhan raid has Medivh cheat every so often by doing things like setting your pieces on fire. The game explicitly tells you that Medivh is cheating, though.