For me, depending on the situation, it's kill stealing. Now, I don't mind people KSing from me if it's accidental, once every so often, or in a situation where it's necessary that someone, hell, ANYONE, gets the kill fast. (e.g: In gears when I have downed the leader on a guardian game and his buddy is like, right next to him about to pick him up.)
However, I have seen a growing trend of when me and someone else down each other, my teammates (only happens when I play with randoms) will happily run past me, steal my kill and THEN come and pick me up. And then run around stating how many kills they've got. Even though they have about 13 kills with the score you get for about 3 kills.
Another that pissed me RIGHT of, there was some stupid noob on Gears (yes, I play gears a lot, I don't know how I haven't actually snapped and gone on a REAL rampage) was on my team, and I had a team of 3 with me anyway. We saw this guy had a mic, so (being a generally friendly until you piss us of bunch), I hopped into game chat (closely followed by the rest of the guys). What we entered into, was a 12 year old little screeching american (no offense to americans who are not 12, or who don't screech all over the mic), who was calling BS and lag every time he got killed (even though the game was fine for the rest of us). But we stuck about in game chat, tellin him to calm down, and trying to tell him where he would be most effective while playing with us (theres no point in everyone going to the same place unless you're on a rush...right?) And at the the start of the second round, he just calls "oh, I'm killing leader". So we're all just like "yeah, whatever man" And my mate that has made leader on our team goes and tags himself in and sits back to relax for 10 minutes. I made a manouvre I am particularly proud of, and ended up (after taking out the 2 guys he had protecting him) in a little 1 v 1 with the enemy leader. Now, I knew this kid wanted him, so I was getting ready to back off, when I checked and saw the kid was MILES away, and there was no way I was gonna last that long, or that his cover would stay down that long. So I did what I had to do, and killed him. This kid then proceeds to spend the rest of the entire game (which ended in 3-2, so thats a LOT of rounds) shouting, swearing, and saying how I stole his kill "coz i called it man!!"
Sorry about the WOT and rant there

but it IS relevant.
Another nice short one, is in Forza. Now fair enough, occasionally I do clip people on my way past if I cut it too fine. But there are some guys, and I swear they must actually drive straight AT you, rather than for the corner, and they send you spinning off, knackering up your sexy cars!!