See lower part of your body
From Halopedia, the Halo wiki
The See lower part of your body glitch be done on any level with a Scorpion, as well as the single-player maps Metropolis and The Great Journey with the Scarab.
Scarab Method
On Metropolis, play until you reach the part where you jump onto the scarab from the bridge. Jump onto it and then wait until it stops.
After it stops, go onto the right or left back leg of the Scarab. Walk into the body of the Scarab and move back a little.
Look down to see your legs moving without you.
This is basically the same with The Great Journey except you must fly onto the Scarab in a Banshee. fly with the Banshee onto it.
Scorpion Method
First, you must get the Scorpion onto a sharp incline, such as the rails opposite the bases in Waterworks, or between an edge and the ground. You must tilt it so that the cannon is facing upward.
Second, get onto the body of the Scorpion. Walk under the cannon, crouching, until you see a cross-section of the cannon. If done right, your legs will walk out from under you and keep moving so as to walk forward, but they are stopped by the joint between the body and the cannon of the Scorpion.
Note: This method is much harder than the Scarab method.