From Halopedia, the Halo wiki
The elevator glitch is a Halo 3 and Halo 3: ODST glitch that allows players, with a partner, to get to very high places. This can be done in Matchmaking, custom games, Forge, and Campaign, as long as you have a right angle.
- Find a partner and a 90 degree corner that extends upwards higher than around six normal jumps.
- One person should go and look directly into the corner, than walk straight into it.
- The partner should crouch on the first person's head and do the same.
- The person on the bottom should start pressing A in a rapid, repetitive way, making sure that both people are looking into the corner and walking into it the entire time.
- If done right, you will go up until you reach the top, and your partner will be in the, supposedly, "unreachable" spot.