When one player uses the knowledge they have at hand to make the other player's characters look like fools for no reason.

For instance, one player makes their character make a suggestion, and is supposed to be reasonably good at what the suggestion is about. The other player makes their character absolutely tear the person's argument apart and makes the other character out to be an idiot. Really, any post that somehow states that the actions of another player is somehow stupid when the player who made the post did not intend their character to be stupid at that. It makes the character break character in another person's post, due to what might have been a minor lack of knowledge from the player's part. The character in the game should not suffer being called a fool because of a minor mistake of knowledge in their player or whatever.

When that happens, it irks me to no end. Specifically, the person who made the choice of making the other character look like an idiot when it clearly wasn't the intention. Yupp. It makes me want to punch that guy.