Corpse Respawn

Corpse Respawn is a lag-induced glitch in Halo 3. The glitch causes killed players to respawn as corpses in multiplayer matches.

Description
A corpse-spawned player acts and behaves almost like an ordinary dead body: they cannot move, shoot, or perform any other action. However, they can look around, with the camera anchored inside of their head. Other players can hear the affected player on proximity voice (if said player has a microphone), and the player's gamertag is displayed above their body when another player aims at them. The player will also see "Hold RB to..." messages on-screen, though holding RB will have no effect.

It is possible for players in this state to actually pick up weapons. If a glitched player is meleed or otherwise moved onto a weapon, they may actually pick up the weapon; on the glitched player's screen, they will appear to be holding the weapon normally, and to others, the weapon will appear to be glued to the corpse's hand. In some cases, a glitched player holding a weapon may be meleed into a position where they can see their weapon in two places at once: as part of their HUD and in their cold, dead hand.

Furthermore, such corpse-players are regarded by the game as living bodies -- their deaths don't count as, well, deaths. If, for example, a player corpse-spawns in an Infection game, then they will become the Last Man Standing, and the game will not end until their corpse dies through some other means (specifically, by falling to its death).

Other odd quirks include the inability to launch glitched players off of Man Cannons; meleeing a corpse-spawned player onto a Man Cannon will cause them to launch out of the cannon before quickly warping back to the Cannon. Furthermore, it may be possible to separate a corpse-spawned player's camera from their body.

Explanation
In Halo 3, SPARTAN-IIs and Elites are normal objects, defined as tags; this is why it is possible to place inanimate player models when modding map variants.

When a player spawns, a player model is placed on the map for them, and their camera and controls are bound to that model. When they die, a new model is created for them, and their camera and controls are bound to the new model.

This glitch happens when the game lags. It is possible that the lag prevents the creation of a new player model, causing the affected player's camera and controls to be rebound to their corpse -- it's still a player model, albeit an unusable one.