Glitches

Glitches are either errors or bugs in any program (game, software, etc.) that can cause undesired or interesting results. In relation to Halo, glitches can be exploited in many ways to make the game interesting or annoying.

Halo: Combat Evolved Examples
Not many were present in Halo: Combat Evolved, however there was the occasional glitch that allowed you to ride a Pelican, skip a certain amount of a level, or get out of a map. There is also a Marine that copies himself when dead. Also, there is a glitch in the end of the level Halo in which the final structure has Marines and two technicians. If you kill the Marines, the techs will curl into fetal position. If you stick one with a Plasma Grenade while in fetal position, they will stand up, spread their arms, drop their weapon and start spinning, before being killed by the grenade. And on the multiplayer map named Death Island(And maybe in the one-player version)You can drive out on the sea surronding the island with any vehicle!

Halo 2 Examples
In Halo 2, many glitches resulted from unforeseen actions the developers did not think could occur. Players could do things such as Super Jumping or Super Bouncing, get past Invisible Walls to get out of maps, etc. "Button Combos" also existed, such as one that would allow you to kill a player instantly.

In the level Delta Halo, there are perhaps more glitches than in the entire Halo trilogy put together. For example, there are many cracks and errors in the invisible barriers surrounding the playable areas of the level that enable the player to skip an amount of the level, explore the distant hills, and even get under the lake, on foot or in a Ghost.

Halo 3 Examples
Halo 3 has been greatly combed over for glitches by a considerable amount of players, but Bungie has put a lot of effort into making sure that there are few, if any, in Halo 3. There are a few that do exist in active gameplay though, such as the Mine Glitches, floating objects, driving an AA Wraith, going under the playable area of Construct, Fusion Coil glitches, making Elephants fly, and the Elephant speed boost. Click here for a list of several glitches found in Halo 3.