Oh boy, I've got quite a list.
1. Telling me they're not making it after the start time, or telling me over Facebook, even though I've told them my phone doesn't have Facebook and to just text me instead. Or not telling me at all. Thanks guys.
2. Arguing that if they had known something bad might happen, the wouldn't have done what they were doing. I had this one come up with one player a lot. One time he abused someone to their face in a foreign language, then reacted badly when they caught his meaning and took offence. What did he think was going to happen? I don't know.
3. Arguing about the fact something they tried only sort of worked, while using a degrees of success rules system , where that's always a possibility. This is really an extension of 2 in a lot of ways. Same player of course. Eventually I learnt to never to use the words 'passed' or 'failed' and just took to telling the players what happened.
4. Making or trying to make the same character every game. The stealthy sniper assassin lady. The in your face, shirtless, long-haired bisexual who hits on everyone he meets, without regards to how uncomfortable it makes some players, or the fact it doesn't make sense in setting since it's the 1930s.
That's another one:
5. Roleplaying characters that upset or might upset other people. Roleplaying is a varied hobby and you never know which of your players might be homosexual, or trans, or have past traumas. Some of us are just a bit uncomfortable about certain topics. Do you really need a character with an abusive sexual relationship as backstory? Will it enhance a relatively light hearted and idealistic campaign, or bring down the tone?
Right, now after taking it to a dark place, how about something a bit sillier. Also throwing fuel on the fire.
6. Players taking notes. Please don't, not in my campaign anyway.
I run a super improvised theatre of the mind campaign, attempting to constantly keep the plot moving over the 2-2.5 hr session. Players trying to take notes is nice and all, but it means they aren't paying attention to what's happening right now and the right now tends to be the bit that kills people. It also generally affects their roleplaying.
It also sucks for me, since someone who's written down notes tends to be someone who gets hung up on little things like the names of characters or places changing when I forget and have to make it up on the spot. I generally run super simple adventure of the week games, with a basic overarching plot, so all they really need to follow is the overall thread.
This one is a side effect of the style of game I'm running of course and it definitely helps that I've got an a good memory for the plot and setting myself. Some of the more... meticulous players I've played with have had real trouble with this, especially since they usually have to, but they got used to it eventually.