My pet peeve is that for the longest time, my sister would eat cheerios with her mouth wide open. That stuff smells like ass when it is consumed. Whenever she eats it I have to leave the room, it disgusts me.

Also, people who think MS Paint is a legitimate image editing program. I had an acquaintance who was in charge of the high schools literary magazine, and he did the majority of his work in MS Paint. It was aggravating to say the least.