Whenever I team kill, it's always accidental. Unless I'm avenging my death by a teamkiller who specifically meant to kill me. Here are a few anecdotes:
1)I was sniping, finally got my crosshair on the enemy's face, I click the trigger... a teammate gets in the way. No, it's not like his leg got in the way, he was running by, and his head was in the way. In THE DISTANCE, close to my target. I was pissed, cus my target ran away after I finished reloading.
2)So I'm exiting the base and heading towards the enemy. This guy walks up to me and knifes me. He was on my team, but I figured he could've just been trigger happy and didn't notice we were on the same team. Okay. I respawn at the base, start leaving, and I'm knifed down again. Same guy. I'm a patient guy, so I respawn again and start to leave. Knifed down again. I respawn and leave a different way, knifed down again. I respawn this time and start looking for him, and Shoot him on site. Of course, he's evasive, so it took a few more seconds than usual to gun him down. Feeling accomplished, and down 25 points (-10 team damage, -15 team kill), I start out of the base, and there's an enemy tank heading my way. Considering I'm not equipped to take it out, I hide in a building and wait for my teammates to take it out, throwing a grenade or two from time to time. I exit the building and head out of camp, and the same loser that was knifing me before gunned me down.
(both events were on Battlefield: Bad Company)