Ring of Rockets
From Halopedia, the Halo wiki
Ring of Rockets is a glitch that can be performed in the Halo 2 multiplayer map Coagulation.
Instructions
On Coagulation, set the game type to just rockets. Then, have a person ride in a Warthog to the hill that is completely covered in shadows. Have another person lock-on the Warthog with a Rocket Launcher, but instead of firing at the vehicle, fire off to the side a little bit so that you don't hit the Warthog and the rockets will begin to revolve around the vehicle. You can fire as many rockets as you want to make a bunch of them circle the vehicle. Eventually, the rockets will disappear. If the person in the Warthog is in the passenger seat, he cannot get out until the rockets disappear or someone else gets in. When you look at the flying rockets you will notice something peculiar. The rocket is aimed directly at the Warthog, but it goes around in circles. Also, if you get into the path of the rocket, it will pass right through you. If the person in the Warthog drives away, the rockets will follow him, but they will probably disappear and not hit him. If the person in the Warthog gets out while he is still parked, the rockets will lose their tracking ability and will not hit him. However, this glitch is not limited to the Warthog; it also works on Ghosts and Banshees.
Why this Works
This phenomenon takes place because a rocket that is homed on something can only take turns that aren't very tight. This causes the rockets to circle the Warthog if they are shot at the right place. This trick can be performed with any other vehicle in any other level, as long is it is in a high point so that the rockets do not hit the ground. A good level to try this is with a Banshee on Ascension. Fly the Banshee into a corner side rail (so it doesn't move) then lock onto the Banshee. Then aim above it and shoot. The rockets will become a ring. Move the Banshee out of this position to make to rockets follow you.
Halo 3
A simular glitch in Halo 3 can be performed with the missle pod.