Unlimited enemies

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

Halo
343 Guilty Spark (Level) Keyes (Level)
 * In the ending swamp area, Flood Combat Forms will spawn continually until you finish.
 * If you don't jump down from the ship into the coolant, Flood will continually spawn from the surrounding hallways.

Halo 2
Metropolis (Level)
 * When you are offered control of the M68 Gauss Cannon, Jackal snipers will continue to spawn and run to the bridge until you occupy the alleyway from which they come.

The 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 usually 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 coming.

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.

High Charity (level)
 * At the end of the level, after going up the second grav lift, the Flood are unlimited.

Halo 3
Halo (Level)
 * When you reach the first battle scene in Halo (the level) with the Flood (where Flood Dispersal Pods land), there is a large beam that rises out of the ground and slants up towards the Control Room. There is an unlimited amount of Stalker Forms scaling this beam.