People who care too much about RSVP's. It was for lunch tomorrow and I actively participated in the discussion about where we should go, but when the organizer sent out the final rsvp list I was not on it and there was a not so subtle and passive aggressive "...assuming participating in the discussion about restaurant does not constitute an rsvp." Why would I continuously offer my thoughts on the details if I wasn't planning to go?
Along the same vein, I got chewed out a year or so back because I responded to an rsvp with "I probably won't be able to make it." Apparently that was insufficiently definite or some horseshit. I don't understand that personality type, and my suspicion is that I never will.
Just to offer a personal anecdote as to why people might care about getting a firm RSVP answer, I once organised a meal for my co-workers. They'd all said they'd probably be able to make it. On the day of the meal nearly all of them had one reason or another why they couldn't go after all and I had to go to the restaurant and tell them we wouldn't be eating there after all and the guy at the restaurant was really upset and guilted me into eating there right then! He said he'd laid on extra staff because of the booking I'd made. I felt really bad and ended up calling round all my friends and getting one of them to join me (and paying).