What's the most uniquely annoying thing you've seen someone do?

'Uniquely' as in: you've never seen anyone else do it and so... so damn annoying... like, if you were a maid at an office and someone had this habit of rolling little of tape and dropping them all over the floor so you had to pick them out of the carpet. Or maybe, this kid you knew would always mock the actors under his breath during a movie... just... IRRITATING...