2. When someone from the group's girlfriend/boyfriend/partner is playing D&D for the very first time and they play a Chaotic Neutral Thief that tries shoplifting torches from the general store in the home base town, and running off to explore in the middle of combat, and you can't regulate or decapitate their character due to "diplomatic concerns."
3. Someone who has never played D&D and "Just wants to watch a game"; It is the worst fucking thing ever as they have always been completely incapable of not continuously disrupting the game, never shutting up with their interjections or exclamations of, for example, "I just don't understand what's going on!" ad infinitum. My rule #1: If you're not playing...you can't be in the room" Oddly/counterintuitively enough, playing in public in a coffee shop has completely hassle/interruption free in my experience. If you're going to be asking a million questions...you might as well be playing.
4. Players wasting time hunting for their misplaced die instead of grabbing one of the many suitable polyhedrals belonging to other players within reach and getting the roll out of the way. This is especially egregious when someone can't find their D20...IT SHOULD BE "FAST DRAW READY" ALL THE TIME!
5. Players that do math out loud: Player: "I rolled a 16", DM: "You hit", "Player "With a +2 bonus for strength that's 17," DM: "You still hit...", Player: "And with +1 for my magic sword that's 18", DM: "YOU MISS!! ROCKS FALL..YOU DIE!!"
6. Players that cast a spell before being familiar with or reading the spell description, Player: "I cast Enlarge on myself and than pick up the castle!", DM: "You are 40% larger...you now are 7 feet tall", Player: "So do I pick up the castle?" RTFSD: Read The Fucking Spell Description
7. Player that never quit whining about wanting to use something from a splatbook in a core-only game, or something that has been houseruled out of the game; "Without a fullblade my character is useless!" ... "Without a spiked chain my character is useless!"..."If I can't play a Psion my character is useless!". This is especially awful when the player is new to D&D and they have a "Grima Wormtongue" munchkin cheese powergamer telling them what kind of character to play.
8. When a player tries something that would never, ever work in the real world* and get huffy when it doesn't work the in the game. Player: "I tie one end of the 100 foot rope to the bridge, the other end to my ankle, jump off the bridge, and attack the plesiosaur in the gorge 100 feet below with my sword!" DM: "Are you sure you want to do that? It will take more than one round to get your rope out and tie it to the bridge and yourself...", Player "WHAT! Jesus christ...that's so unfair...I already had the rope out!"[a couple of rounds later] DM "You plunge off the bridge 100' feet, and than with a horrendous jerk you take 16 points of damage as your leg dislocates from its socket before the rope snaps plunging you into the rapids with the plesiosaur", Player 'WHAT! THAT'S BULLSHIT!!"
9. When Players "ADD Teleport" while in town. Player #1 "I go to the brothel and spend all night with the classiest doxy," Player #2 "I go to the alchemist's" DM "Character #1 is charged 200 go for the night, Character #2 is offered some potions of healing..." Player #1 "I BUY ALL OF THE POTIONS OF HEALING!!!"
* Yeah, I know it's a fantasy game about dragons and magic...but I've got my limits dude... EDIT: And the above example would have a decent chance of succeeding if I was Journey-Mastering an Encounter Critical game!