From Halopedia, the Halo wiki
The sliding Flood glitch is an animation-related glitch found in Halo 2's Campaign level Quarantine Zone. The glitch, which works on both the original Xbox and the Xbox 360, causes the corpses of some Combat Forms to slide when shot. Some Combat Forms will slide a lot, while others will slide little. Once the Combat Form stops sliding, the player cannot make it start again without reverting to the last checkpoint.
To perform this glitch, you need to bring a Shotgun to the end of the level Quarantine Zone. When entering the Library at the end, stop before triggering the cutscene. Other projectile weapons, like the SMG, can be used, but their success is not guaranteed.
- Use the Shotgun to shoot at the left leg of a dead Sangheili Combat Form.
- The game then should glitch, and the Combat Form will make a noise as if he is alive and slide a bit.