Infinite Marines Glitch
From Halopedia, the Halo wiki
The Infinite Marines Glitch is an exploit in Halo 3's Campaign. The trick allows players to spawn large numbers of Marines. It is widely considered a glitch, though technically it is more of an exploit.
On The Storm, after defeating the Scarab and moving through the first aid station, you will enter a storage room, where several Marines are fighting Brutes. Defeat the Brutes and proceed to the next room, where you will find several factory workers fighting more Brutes. Upon entering this room, let all of the Marines and factory workers get killed. When they all die, five more Marines will spawn in the previous storage room and join you. If they die, more will spawn; the cycle can continue indefinitely.
Among other things, you can lead the Hunters back to this area and have them slaughter the Marines endlessly, or you can simply betray them. The Tilt and Mythic skulls make the Brutes and Hunters nearly invincible, guaranteeing the Marines' deaths. This trick can be useful when using those skulls or when playing on Legendary, as infinite Marines will eventually kill the Covenant forces.
The glitch ends when you go outside.
Also, when you're fighting the Scarab, the Marines firing the mounted Missile Pods on the catwalk leading to the first aid station will continually spawn if killed. If you're playing on Co-op, have one player go to the door where the Marines come out of. Then have one player kill all the Marines outside. The door should open. Go inside. The Marines will come out of another door. Go inside. They will spawn between this door and a closed door.
When playing The Covenant on Co-op, a similar exploit can be used. When you get the Hornet and are flying to the third tower, you will see an island with an Anti-Air Wraith on it. Destroy the Wraith, land on the island, and drop your Hornet into the water. A Marine will fly another Hornet to the island and land; drop it into the water. This will cause more and more Marines to enter on Hornets, with one Marine per Hornet. Eventually, though, the sheer number of Marines may overload the map.
Also, if you get the Marines to pilot the Hornets rather than dropping the Hornets into the water, you can have an unlimited number of pilot controlled Hornets.