Stopping time

In Halo: Combat Evolved there is an interesting glitch informally called Stopping Time, that takes advantage of the way that different areas of the levels are loaded into memory.

Overview
Each level in the game is divided into sections that are loaded individually as the player crosses a specified boundary. It is most recognizable by the split-second lag and the phrase "Loading...Done" that appears on the side of the screen. There are various game elements that are confined to these areas; for instance, in many parts of the game, the Marines accompanying Master Chief will hold back when he enters some areas. To accomplish this, the game confines them to certain areas, blocking them from entering others. The same is true with enemies.

Doing the Glitch
Usually the game's design prevents the player from forcibly moving the Marines/enemies from one area to another. This can be accomplished, but usually requires a Warthog to do so. One place that this is easily accomplished is in the level Assault on the Control Room, where Master Chief and the Marines fight their way through the deep canyons into a dead-end with the two final bridges high above them. The only way out of the canyon at this point (at least, according to the game's intended design) is through a single small door on the right side of the canyon. The Marines will not follow, instead hanging back to guard Master Chief's back.

While it is somewhat difficult, a Warthog can be forced into this area of the canyon with a little effort through the pillars that block the way. If the player fills the two extra seats with Marines and then forces the Warthog through the door into the corridors, once the Warthog crosses the loading point the two Marines will freeze. The reason behind this is that when the player leaves one area, all the "living" elements in that area, whether NPC, Covenant, vehicle, explosion, etc., are frozen where they are, even if they are taken out of that area. You can shoot the Marines once they freeze and while they may make sounds, they will not move until you return with them to their specified area.

Another example of this glitch can be seen in areas where Hunters roam the interior rooms (in the same level). If you can attract one to follow you into a corridor that contains a loading point, once you cross that point the Hunter will freeze. However, firing into the frozen area or throwing a grenade into it will not freeze that bullet/projectile or grenade. Instead, it will act normally, and if you've damaged the Hunter blood will even splatter on the floor and walls. If you damage it enough while it is frozen, it will die while frozen, but not fall down until you re-enter the frozen area.

This glitch takes a lot of experimentation, as there are many other interesting sub-glitches that can follow.

Trivia

 * This glitch is very useful when you are in a great need of ammo. Make the game freeze, kill the frozen enemies, and then pick up the grenades and ammo.