Sometimes it's kind of hard NOT to hit a teammate when they are being overun by zombies. It's either try and shoot to help or do nothing. From now on, I'll let you get torn apart! Hehehehhee! 
I didn't mean when your teammates are getting swarmed... I'm talking about, when your walking up some steps and your teammate fires a round into the back of your head for no reason. This continues for 2 hours and they keep "accidentally" shooting other teammates.
But I don't think that is as bad as not playing as a team. If one person is running around trying to horde all the pills, med packs, pipe bombs, etc and uses them like an effin idiot (i.e. Uses a bottle of pills, then immediately uses the med pack), it just doesn't work out, at least not on expert.