Unlimited enemies

There are certain points in the Halo games where it is possible to face unlimited waves of enemies in Campaign mode.

Halo: Combat Evolved

In the ending swamp area, Flood combat forms and infection forms will spawn continually until the player approaches the structure.

At the first large pool where player can pick up a Sniper Rifle, infinite Flood carrier forms will always comes out from it until player made past it through.
Near the gravity lift where there is a Shade Turret with some Covenant forces guarding it, multiple Flood Combat Forms will continue spawning and attack the encampment until player made past it through.
Inside the ship, in the corridor before the cargo bay, infinite Flood carrier forms will drop down from gaps in the ceiling.

In the Engine Room, Flood combat forms, infection forms, and Sentinels will spawn continually until the engines are destabilized.

When the player is offered control of the Warthog's M68 Gauss Cannon, Kig-Yar Snipers will continue to spawn and run to the bridge until the alleyway they are coming out from is cleared.

At the beginning of the level, when the player gets to the first elevator (after meeting the Heretic Leader's hologram), get on it and get off immediately to face four Sentinels. An endless amount of Flood Combat Forms are climbing up and jumping on to the elevator. It is imperative that the elevator does not go down.

After acquiring the Scorpion Tank, fight through the level. When D77-TC Pelican drops resupply canisters, Up on the structure where all the Covenant shields are, there is an endless supply of Unggoy. They appear from the middle of the structure about three or four at a time. Once the player leaves the tank, the supply will end.

Inside the Temple, if the player does not interact with the Prophet of Regret, an unlimited supply of Unggoy and Sangheili Honor Guardsmen will enter through doors to protect the Hierarch.

The Sentinel Launchers at the beginning have an unlimited amount of Sentinels to launch.
After de-activating the Containment Shield, if most of the Sentinels in the gondola area are kept alive, an endless supply of Flood combat forms will enter the area.

Inside the first structure, where Ghosts will continuously attack the player, an unlimited amount of Ghosts will enter the structure, fighting the player.
The outdoor area after meeting Needler Sentinel has an endless amount of Sentinels.

At the end of the level, with the conduit leading to the Forerunner Dreadnought, an infinite amount of Flood combat forms and later, carrier forms will spawn, eventually overwhelming the limited Jiralhanae force in the area.

The large beam that rises out of the ground and slants up towards the Control Room has an unlimited amount of Flood stalker forms scaling it.

At Airview Base, an unlimited amount of T-52 TC "Phantoms" dropships will arrive and hover over the area until the M71 Anti-Aircraft Gun is activated. They can be destroyed for a large amount of points.

After the player has fought through the Covenant on the beach and reached the Sabre Program facilty's entrance, T-25 TC "Spirit" dropships will continuously drop off more Covenant to assault the beach. UNSC Pelicans will also fly in to drop off squads of army troopers.

At any point in the level, continuous squads of Banshees will fly in and out of the map. The occasional Phantom will also spawn, as well as a friendly Pelican or Falcon.

In the final landing pad area, if the player manages to reach the Mass Driver that Emile-A239 is using by performing a glitch, he will be able to use the Mass Driver to shoot down infinite Banshees and Phantoms in the distance.