Infinite Marines glitch

The Infinite Marines Glitch can be done on the Halo 3 level, The Storm.

Walkthrough
After defeating the Scarab and going through the aid station, you will come across several Marines fighting some Brutes in a large storage room. Defeat them and go into the next room, where you will find several Voi Factory Workers fighting the Brutes. When you enter this room, let all the Marines and factory workers get killed. After they are all dead, five Marines will spawn in the previous storage room and join you. If they die, then more will keep spawning indefinitely. You might need to walk deeper into the room, near the Brute holding the worker, in order for the Marines to join the fight or they won't follow. You can lead the Hunters in the next area back to this storage room and watch them destroy countless Marines or you can start betraying them. You can turn on the Tilt and Mythic Skull, the Brutes and Hunters will be nearly invincible. The glitch can be very useful on Legendary because infinite Marines will eventually kill off the Brutes and Hunters. The glitch ends when you go out outside.

Also, on The Storm when you're fighting the Scarab, the Marines firing the Missile Pods on the catwalk leading to the aid station will continually spawn if killed.

The Infinite Marines glitch can also be found on The Covenant, though only on Co-Op. When you get the Hornet and are flying to the third tower you will see an island with an AA Wraith on it. Destroy the Wraith, land on the island, and drop your Hornet into the water. Another Hornet will come and land, then drop it into the water and another will show up. Each Hornet carries one Marine and will keep spawning, although the map will overload eventually.