Crumbs in the bed! (Admittedly, they're my crumbs, but they're still annoying).
The supermarket carpark was full today, so I waited in my car across the road until I saw someone walking to their car with their groceries. At that point, I quickly drove over there and patiently waited, with my indicator on, for them to leave. No worries; they nodded and waved, reversed out - only for an old fart in a convertible to dart into the empty space faster than I could put my car into gear! I'm trying to be more relaxed about things so didn't challenge him, but I was really annoyed. Why do people do this sort of thing? It's not like he wouldn't have seen me.