The guy who was on my team in the days I still consented to join my friends on Halo 3. My friend was running around with a turret, killed about 5 enemies with it.
This guy obviously decides that he can do better and assassinates his own team-mate, with a quick crack to the back of the head. Then, proceeds to run off with the gun.
Now, the team I was on was made up of that guy, and myself and two friends who were in the same room as me. We decided we weren't having this, and so Xbox Live didn't consent to administer justice, we became vigilantes, equal in ferocity to the punisher.
After killing the guy twice each, Xbox Live thought it might be time to step in and boots all three of us. And that my friends is irony, or not if I'm just being lazy.