Yesterday I got shot in the back twice while fighting a naked guy with a club. Not because the fight lasted that long, but because 2 archers decided I needed help (kill steal).
Anyway, most of the time I have no trouble with team hits other than accidents (swing at an enemy, he dies just before your swing connects and you hit a teammate right next to you).
I hardly ever get killed by stupidity, but then I make sure I stay away from anyone swinging long things.