Based on your other thread, I think your use of the word "glitch" may be causing your responses to be targeted differently than your intent. "Glitch" has a more benign connotation to it than other similar words such as "bug" or "defect." Glitches are going to be the more random (i.e. non-repeatable) errors that may have an effect on gameplay but are usually minor, infrequent, and often funny (such as the video posted with FIFA and the very...well-acquainted...players). As glitches are often extremely random, it's unlikely that they would be well documented as persisting from one year to the next. For instance, I would challenge anyone to recreate the scene in FIFA in any version of the game (including the one it was recorded in). In the end it's probably not even a true bug but just an interesting case of how physics are handled and how things aligned perfectly to create that situation.
If you asked for the worst "bugs" or "defects" I think you would get a more targeted response on things that are repeatable, exploited, and often have an undesirable effect on the game. These would be things like guaranteed goals using a certain strategy in the NHL games, always getting a sack as a previous poster mentioned, game crashes/freezes, incorrect scoring or game rule implementation, etc.
Perhaps the worst offender would be design flaws -- in this case, it's likely not a bug that causes the undesirable gameplay, but rather the game just being poorly designed. A good example of this would be with Michael Vick in Madden '04. There was no "bug" in the way he was coded, but the unintended side effect of his ratings and the gameplay was that he was virtually unstoppable because of his running ability and decent passing skills. The game's AI just wasn't built to be able to contain a quarterback with his particular skew of ratings.
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