My peeve? Being called in to do a presentation for, say, 15 people, to find that it is scheduled in a room with barely space enough for 10, when you consider the chairs. Oh, chairs, you say? Well, we've got 7. Everyone else can just stand up for four hours straight, right? No?
Then, everyone stares at me, as if I should be able to solve the problem for them.
Really, people. I do not carry chairs in my pocket, nor magical expanding space. It's your facility, you should know how many people it can fit.