From Halopedia, the Halo wiki
On the Halo 2 multiplayer level Colossus, there is a glitch that will allow you to get out of the map. To perform it, go to the bottom of the level where the windows are (which the orange sky can be seen through the glass). These are the angled windows flanking the Overshield, not the ones directly next to them. Then go into the very right or left side of the window and then crouch and go as far into the corner as you can. The next part may be difficult for some players, as there are two ways to do this.
What you do once you are ready is quickly jump three times in a row while looking up. If done right, you should see a flash of light. At that moment you move left or right (depending on the side you were at) and you should fall through the window.
In this method, keep crouching and press A many times, then walk onto the glass part, and you should fall through the glass out of the level (This does not work as well as the above method).
Once you are out, you are stuck until the round ends, or you commit suicide. Also, while here you cannot be killed, unless your enemy aims at the very top of your head.