Floating Mantis glitch
From Halopedia, the Halo wiki
In order to perform this glitch, a Mantis must be spawned, on any map. It must then be destroyed by the guardians. This may be achieved by moving the Mantis in to a kill-barrier or a kill ball. After it is destroyed, the Mantis will then slowly float down to the ground, ignoring regular gravity. It will continue to fall like this regardless of being shot at and stood on; however, because of the geometry of the Mantis, it is slightly difficult to stand on it.