There are few glitches in Halo 3, but in those that do exist, many of them are triggered by the Trip Mine. Note- Almost all of these Glitches are to be performed in Forge. However, if you save you map with the following instructions for the Basic Mine Glitch, you can still use the glitches in custom games.
Basic Mine Glitch
This is how it was all started. The following can be used to makes walls of mines, piles, and to be used on the below glitches.
1. Select any map you wish to use this on in Forge.
2. Spawn 4 Trip Mines and set them to respawn in 0 seconds. (Do this by setting your run-time minimum and maximum to 4)
3. Make sure you have a secondary weapon of any type.
4. Stand on the pile, look were you want the mines, and press X and Y at the same time.
It may take time to get used to it, but X and Y do need to be pressed at the same time for this to work. Try doing the sequence at a steady pace, it does not need to be pressed in a fast manner.
Overload Snowbound Glitch
This glitch can be preformed on Snowbound. This glitch will turn the power on the entire level off, so the guardians won't shoot at you when you leave the level. First go to the room were the shotgun respawns and throw the trip mines where it respawns. Throw them until the big blue gate infront of you vanishes.
Overload Elephant/Guardians Glitch
This is performed on Sandtrap. Do the following instructions above, except place the mines NEAR the Elephant, not on it. Once you are ready, keep throwing mines near the Elephant. Once you throw enough, the Elephant will overload and vanish, as well as the guardians for the minefield. Note- Once the mines detonate, the Elephant will reappear, but the guardians are still overloaded for that match. Note- You may also filp the Elephant using about 30 mines(35 to 40 usually makes the Elephant vanish) if you wish, just be sure to be far away and place them close enough to affect it. Once you have done that, shoot the pile when you are ready. Clap clap clap your hands in the air Danny X FAILS GAWK
Overload Gravity Lift Glitch
Can be performed on any map that has a Gravity Lift, for example Construct. Do the Basic Mine Glitch, and once you are ready start throwing mines near or in the grav lift, it does not matter. Once you have throw enough, the lift will overload and vanish. This applies for all the lifts on any map. This can be useful for deactivating them in say a game like Infection. Note- With this glitch performed in Construct, it is possible to get under the map once the lifts are gone. Just go to one of the purple lifts and spawn a Teleporter under it to achieve this. You can also overload other things in a level, such as the shield barriers in Snowbound
Other Uses
This glitch can be used in custom games, just be sure to save a map that has the mines placed somewhere. During a game you can make a wall of mines, disable lifts to make a game harder, or just die in a spectacular fashion. Whatever you can come up with with this glitch is probably possible.