Confusing your house rules with the actual rules and then getting belligerent, particularly when discussing problems with a game.
Joe "Game X has Rule Y, which isn't very good."
Bob "No it doesn't. It has Rule Z."
Joe "I'm looking at the rules right now, and you totally made that up."
Bob "Well, technically, yes, I did. But I think its a good rule, and I'm going to abuse you for not knowing about it."
Sheer bloody minded provincialism.
Joe "Game X has (or lacks) Y. Any game that has (or lacks) Y will have Flaw Z, or will appeal only to players with horribly negative traits."
Bob "Dozens of games have (or lack) Y, and don't have those problems. Here is a list."
Joe "Given the choice between believing you, looking up those games, or blindly slandering tens or even hundreds of thousands of people I don't know, I choose slander."
When editions change, or when people consider new games, they pay a lot of attention. This means that things that often happen behind the scenes become more pronounced. Sometimes people react to this by becoming very, very upset about something that already existed in their original game, but which they previously overlooked.
Joe "It is hideously unrealistic that you can heal a broken pelvis overnight in Game X. That's why I prefer Game Y."
Bob "In Game Y you can heal a broken pelvis in three nights."
Joe "So? You never think about it in game Y."
There are also some really strange, fundamentally weird issues people have with concepts like verisimilitude, the relationship between DMs and their own game, etc. But those take longer to discuss.