First: our group is a group of friends. We've all known each other from early high school and are now all out of college - it's been a while. We haven't been playing most of the time, maybe only the last two or three years.
This being said, we have a player who, while he has a master's degree and generally did better in college than I did, has zero patience with learning any new rules or taking the time to actually learn his characters outside of his one favorite D&D character. This is the kind of guy who claims I nerf everything and idea he has when I GM. He's the guy in the group that, when you explain shooting at the zombie who is struggling with his best friend, he may shoot the best friend on accident, then shoots, and almost kills the best friend, he complains wildly about how "gay" it is and lame. I mean I WARNED HIM BEFORE HE ROLLED THE DICE, TOO BAD IF THE ZOMBIE DODGED IT AND HIS FRIEND DIDN'T!
Anyway, just Friday night we're playing D&D 4E. We're in an inn trying to talk to the city guard. This friend, who's playing a beast master ranger with a polar bear for a pet, decides that, while I and my brother and assisting in a silence rune to protect the room, he's bored and his pet is going to dig a hole in the ground... IN THE INN. The DM, another friend, is trying to get some important plot information to us as I'm trying to role play with the city guard, and we hear "my bear rolled a 34 strength test to dig in the room!" As my brother points out that there's a cellar and the bear will fall 20 feet, taking 2d10 damage, this trouble player exclaims "I didn't really do it!" He does stupid shit like this all the time and no one holds him to it. Not this time, my brother insists since he rolled for it, the bear should take the damage (seriously, the bear has over 100 hp, he can handle the fall for God's sake). This player gets all huffy and the DM agrees, so the bear takes all 8 damage from the fall.
We got over it later, but God, this is a burgeoning problem that is going to have to dealt with some day. The friend's good company when not trying to blow shit up in every occasion in a fake world when we're all playing make-believe.