From Halopedia, the Halo wiki
The jetpack glitch is a glitch in Halo: Reach that causes the player to float in the air indefinitely.
To do this glitch, Start a game of Firefight with one player, or any other gametype with at least two players. Set the armor ability use to unlimited. Player A should get a Jetpack, and an Energy Sword. Player A should hover just above the ground with the Jetpack. At this point, player A must kill player B (or an enemy in Firefight) with the sword while hovering with the Jetpack. This will make the player float up to the top of the map, and then stay relatively still, only able to move by being pushed by explosions or projectiles.
Another variation of the glitch is when you are in Forge, simply jet up with the jetpack and while you are in the air switch to forge mode. If you turn the monitor around, you should see two orange balls of light floating where you where, along with the Jetpack audio.