Telefragging

Telefragging is an unknown factor that occurs during multiplayer gameplay in Halo: Combat Evolved and Halo2. It is a rare occurrence, and usually must be performed deliberately.

This is actually not a glitch, and is in fact an intentional part of the game design, to prevent both the blocking of teleporters, and players teleporting directly onto another player. Telefragging can be prevented either by the player standing on the receiver and moving off of the receiver, or by the player going through the sender backing away from the sender.

Halo: Combat Evolved
In Halo: Combat Evolved, a player must go through any teleporter and remain standing still at the exit of the teleporter. If another player tries to go through the same teleporter, they will not be able to go through. However, the first player's screen will start to go white and their controller will start to shake. Their body will then start to go sprawling away from the teleporter, and, if said player dies, the game will say that Player 2 "telefragged" Player 1.

Halo 2
In Halo 2, by parking any vehicle or standing in a teleporter you can block it. When another player attempts to use the teleporter, they will not be able to teleport and will receive a message "Teleporter is blocked", If a player is standing on the receiver his screen will gradually turn white, similar to the effect in Halo: Combat Evolved, and when the player's screen has gone completely white he will die and it will be attributed as a suicide.