In Halo 2, in the level The Oracle, there is a way for the player to find the holo-drone that the Heretic Leader uses before it is activated. Activating this glitch in Halo 2: Anniversary unlocks the 10 Minutes Too Early achievement.
First, when you start the level, go to the room with four pillars on the right and left side. Jump on the pillar that is closest to the glass tube. This leads to a small square room; in the center of the room, break the glass and jump on one of the four holes in the outer part of the square to get in. When you get inside, it's dark, but there are four glass windows on the top of the square. Break one of them and jump (crouch or grenade jump recommended) on the edges. Then you can see another room on top of the square. When you jump to get there, there are two holes on each of the four corners. Two corners have pillars to help you get to the hole, but the other two corners don't so you need to grenade jump to the hole where it is located. You will find the holo-drone floating on the hole.