While not a form of MW2 noob, kill-stealers are still very annoying. You have a perfect and humiliating headshot for the last kill, and suddenly someone runs directly in front of them and shoots them in the leg. This is still a form of annoyance, happens any time. It can happen as mentioned above, or you are only a bullet away from a veteran or marksman 6 etc. and someone turns the corner and steals the last shot, which ends up to the leg.
While kill-stealers have no in particular classes, there is no way to stop them, except using a more powerful weapon or Stopping Power.