This is almost unexplainable as to why it happens.

Bungie.net : Bungie Online : Halo 3 File Share (SLOT 22)

Requirements: Forge, Host, Foundry.

Step 1:

Go into Foundry, and get through the windows on the right-hand side.

Step 2:

Looking towards the pipes, hit Up then A quickly to Forge as human and spawn a Sender. (Default Channel 0)

Step 3:

Looking towards (inside) Foundry, repeat Step 2 with a reciever.

Step 4:

After 30+ seconds grab the Sender and place it on the ground in an area you can walk to and go through it.

Effect:

The teleporters periodically go on/off and appear in different areas often.

nice find I'm going to try it when i get xbox live