I almost put this in "What little things drive you completely up the wall", but after reading it again I thought it might be a little gross, so I put it here instead.
It's annoying and extremely gross when people don't make sure the dishes are clean before they empty the dishwasher after it's been run. Our dishwasher is old, and occasionally it doesn't do the job. This morning when I was making coffee, I added the sugar, splashed in some half-and-half, and stirred. It was only when I put the spoon in my mouth (sort of a habit after stirring coffee or tea, make of it what you will) that I noticed something was wrong. I felt something crusty on the bottom of the spoon, which turned out to be a large glob of petrified, tasteless peanut butter. Which is icky enough, but for an awful moment or two I honestly could not be sure that it wasn't fossilized cat food! Seriously, how hard is it to actually look at what you're (general you) doing when you put dishes and silverware away?
So yeah, that coffee ended up down the drain because I couldn't get past the mental block of "Ewwww, nasty food gobs in my drink!"