People who, in a roll that's supposed to start on the knees, instantly stand up, before either of you have taken grips or anything.
I mean, standing up's a good idea in a fight if you can, but we're supposed to be starting from knees. If you stand up straight away (without any contact on me etc), then I should probably stand up too, and then we're not starting from knees any more. Which I don't really mind, but that's what we were told to do.
Today I slapped hands with one guy as we were standing before we sat down, sat down into a butterfly position expecting him to sit down too so we could start, and instead of doing so he simply walked over my legs from standing and started from there. I mean... we start from the knees. We hadn't started yet.
Also people who always pull guard at the start of a roll. You start a roll, they pull guard. Someone taps, you start again, they sit back for guard again. And again. The next time, you decide to sit back for guard, and even when it's obvious that you want to start on bottom, they STILL sit back for guard, forcing you to give in and take top position again or continue the embarrassing spectacle of two guys trying to pull guard on each other...