Someone eating cereal in an otherwise silent room. Particularly when I'm trying to focus on something. It's everything. It's the chewing. The milk falling from the spoon into the bowl. When they bite their spoon (which no one is supposed to do). It all makes me irrationally angry.

And I'm not an angry person in most situations.

I had a mission companion who would get a mixing bowl fill it with cereal, and eat it during morning scripture study. Errgh... it was torture.