Stretch
From Halopedia, the Halo wiki
What is 'stretched'?
This glitch makes MC's neck stretch, or sometimes makes his body flop. To do this, go onto the map Midship, and find one of the shields.
How to do it.
Crouch down and keep walking towards the shield, so you don't bounce away. Get somebody to 'assassinate' you. If you do this right, MC's head should go through the shield with his body on the other side. The shield makes the body pull away, but it can't because the head stops it from doing so. This causes MC's neck to stretch like a rubberband. Sometimes it flops around like a fish out of water. Also in Halo 3, when a player dies and hit something violently, theres a high possibility they stretch but however it happens for a split second only.
Also See
Hollywood Halo-neck stretch glitch[1]