From Halopedia, the Halo wiki
Halo: Combat Evolved
The Pillar of Autumn
An unresponsive Elite can be found on the level Pillar of Autumn. During the first close encounter with the Elite that taunts you (Easy and Normal only), run towards the marines, making sure the Elite follows you. Once the doors close, the elite will become unresponsive.
The Silent Cartographer
On The Silent Cartographer, an unresponsive Elite Zealot can be found. In order to witness this inactive Elite, one must use the Warthog to get through the door in the chasm before it is locked. If the player fails to accomplish this, the door will lock and will remain locked until the player deactivates the island's security system. If the player makes it through, the Elite behind the door will not attack. This occurs because the Elite is programmed to stand behind the door as it is being locked and is only supposed to attack once the player unlocks the door and plays through the level naturally.
Assault on the Control Room
In Assault on the Control Room, play through the level until you reach the part where you see Elites taking off in Banshees. Run straight to the Banshees, ignoring any enemy firing at you, or use a Sniper Rifle to snipe them; if you fail to kill any of them, revert to a checkpoint and keep trying. Once you've killed an Elite, take his Banshee and fly down to get a Karo'etba-pattern Ghost. Getting the Banshee is not required, but by using one, you can avoid triggering enemies to spawn on top of the pyramid structure, allowing you to get there faster.
Drive the Ghost all the way up to the top, where you can open the door, but do not open it yet, or you will have done all of this for nothing. Instead, drive your Ghost up to the white triangle, and hover there so that you are directly above it. Make sure that the door is directly to your right.
Now begin strafing to the right and press up against the door. At this point, you should be able to see a Grunt or two on the other side. If you can see your Ghost at least partially on the other side, you are doing it correctly. Once you are completely against the door, exit the Ghost. If done correctly, you should exit on the other side of the door.
There, you will see all of the Elites and Grunts, but they will not attack you. Because you never entered through the door, you never triggered the checkpoint that 'activates' the AIs, so they will not do anything. To 'reactivate' them, just reenter your Ghost, pull away from the door, and activate the control panel, or activate the control panel on the side with the enemies.
If the glitch does not work the first time, do not give up; try again at different angles.
Another, easier method to witness this glitch is to play through the level as you normally would, up until you reach the first blast door at the Control Room's entrance corridor. Bring a Ghost up to the door and position yourself at the center of the door, with the door to your right. Strafe left for a short distance, then strafe hard right. Once you are about halfway to the door, exit the Ghost. If done correctly, the Ghost will continue to strafe right and push you through the blast door.
Once inside, you can have all sorts of fun with the AI that take no notice of your presence. The only way to cause them to react to you is to open the first blast door - the one you hacked through. This can be done by either reentering your Ghost (if it is close enough to the blast door), or by activating the control panel located within the corridor.
In Halo 2, there seem to be many unresponsive enemies; however, they are not unresponsive in the usual sense. Instead, they are only unresponsive to you if their backs are turned - they do not seem unresponsive as much as they are deaf. Here are a few examples.
- The Sniper Jackal facing away from you after you come out of the tunnels in Metropolis.
- The Heretic Elite at the start of the level The Arbiter (Level), after he finishes patrolling (rarer than the Jackal).
- After you kill Regret in the level Regret (Halo 2 level) all enemies will ignore you and run toward the bottom door of the level and stand there, as if there were no enemies around.