Last Resort on Halo 3. There was a guy on a platform with a Spartan Laser. I jumped, threw a grenade just as he fired. He managed to fire his laser, but it missed me and he died. For ages I thought my grenade had actually deflected his laser, because from the replay videos it seriously looked that way. What I didn't notice was that on a little platform below him were four fusion coils. They had exploded at just the right time to save me from being fried. Still epic.
Underpass on Modern Warfare 2. There was a guy crouching behind a wall, and I trained my Barrett 50. Cal on his head. My friend saw the guy from behind, and so ran up to him to knife him. I shot at the exact same time my friend brought out his knife, and I got awarded the kill. My friend said that he saw the blade hit the guy's head as his corpse fell to the ground. He still hasn't forgiven me for that exceedingly well timed kill steal.