Quarantine Zone floor glitch
From Halopedia, the Halo wiki
Between the start of the Shooting Gallery chapter and the checkpoint that follows it, the player will travel via a gondola. The player should drop down to the bottom of the gondola, where they will find a tunnel that several Flood will appear out of. The player should keep pushing forward on the joystick, but avoid actually entering the tunnel.
If the player manages to perform the glitch, it will look as though they had walked to the end of the tunnel and jumped off. They will not land on the bottom of the gondola; they will die. If the glitch does not work immediately, the player may need to "wiggle" around a bit, changing direction slightly as they move.