This could be anything, ranging from how a game is designed, to how it's played.

To me, a game's biggest sin is when a game breaks its own rules to give challenge to the player. What I mean by this is this:

Every game has its own set of "rules" that are set by its design. Everything from AI logic, to animations, to general movement, has to abide by these rules. But there are certain games that essentially break themselves to give the player a challenge. Especially in multiplayer games, where there is no real viable alternative to human intelligence (not yet, anyways). This is usually accomplished in two ways: A) Create a situation in which no person can reasonably overcome by use of pure skill, or B) Abuse and hack the actual game mechanics to counter and negate the player's quick skill.

For instance, take a fighting game like Dissidia. This game ALWAYS breaks its own rules to give the player a challenge. And since Dissidia's MP is ad hoc, you'll end up fighting against the CPU most of the time. It's one of those games that do both of the aforementioned sins. In the Story mode, you'll usually either go up against cupcake AI manikins that are incapable of lasting longer than 10 seconds in a match, or you'll go up against AI that you have to be extremely lucky to beat. At its most ridiculous, you can end up fighting manikins and bosses with 20 stacking, redundant accessories (when the player will certainly never be able to acquire that many), in addition to the cheating that occurs in the actual gameplay vs. the CPU. Madden (and most competitive games in general) also has a habit of breaking its own rules using warping, glitching, defying it's own systems, and of couse comeback AI.

A good example of a game that doesn't break its own rules is Heart of Darkness. It's an extremely difficult game, but that difficulty isn't created by cheating the player. It's created by adjusting various types of situations randomly by switching enemy types and enemy counts with different types of platforming obstacles and puzzle solving. The player never feels cheated when he/she beats a stage, and there is a true sense of accomplishment, rather than a sense of frustration.
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