Unlimited enemies

There are certain points on the Halo 2 Campaign where it is possible to face unlimited waves of enemies, providing you follow certain instructions during the appropriate gameplay section.

Locations
Oracle (Level)
 * At the beginning of the level when you get to the first elevator (it's after you meet the heretic leader's hologram), go on it and get off immediately to face four sentinels. Then an endless amount of flood climbing up and jumping to the elevator. It is imperative that the elevator does not go down.

Delta Halo (Level)
 * After acquiring the Scorpion Tank, fight your way through the level. When you reach the area where Cortana would normally say "They're all pouring out of the middle", stay in the tank, don't get out of it at all. Up on the structure where all the Covenant shields are, you will see an endless supply of Grunts. They appear from 'the middle' about three or four at a time. Once you leave the tank, the supply will end.

Regret (Level)
 * When you enter Regret's chamber, do not kill him, shoot him, or interact with him in any way. You will receive an endless supply of Grunts, and occasionally Elites. It is important not to interact with Regret so he doesn't die and end the level.

Sacred Icon (level)
 * The Sentinel Launchers at the beginning have an unlimited amount of Sentinels to launch.
 * At the part where you first fight the Flood on the gondola, if you keep most of the Sentinels alive, the Flood will keep spawning.

Quarantine Zone (level)
 * At the area where Ghosts are constantly attacking you (after you first saw the Scorpion Tank) the Ghosts will keep coming at you.
 * The outdoor area you go to after you see the Needler Sentinel has an endless amount of Sentinels.

Halo 3
Halo (Level)
 * When you get to the first main area where Flood drop pods fall and you face all the flood.There is a beam that rises out of the ground and go's up to the building. Unlimited Stalker forms are running up that beam.