I have the dubious pleasure of sitting right outside the toilet door at work. Our office is a bit like a long corridor with the kitchen area at one end and the toilets in the middle.
People are always leaving their cups/glasses/plates/cutlery etc on my desk/printer while they go into the loo.
Its not like they can't take them in with them, there's a bit by the sink they could put them on so they wouldn't have the hold them and pee or anything!
It doesn't bother me as much as it used to, but I still think its a bit rude and they get in the way. I bet they wouldn't like it if I came and left stuff on their desk while I 'just popped out to the shop' or soemthing!