For me, it has to be when people move things, and then don't move them back to their original place. I find it incredibly annoying, and forces me to go out of my way to properly place the things back, I also get mad.
[spoiler][b][i][u]KUPO!!![/u][/i][/b][/spoiler]
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When people say [i]car[/i]-mel instead of [i]care[/i]-uh-mel. It's spelled car[u][i][b]a[/b][/i][/u]mel. You don't skip a letter.