Suspiciously dying in one shot in online multiplayer Halo
Solution 1:
Headshot with a sniper rifle, most probably. A plasma grenade stuck directly to you or a melee attack from the back would also do it.
Solution 2:
The CE magnum is overwhelmingly powerful and since it has a zoom you could have been headshot by it, as it could kill you in a single shot (if the other player landed it correctly.).