Phantom Flood whip
From Halopedia, the Halo wiki
The phantom Flood whip is a glitch in Halo: Combat Evolved and Halo 3 where a Flood Combat Form can whip the player after the player shoots off the arm it uses as the whip. It can be performed on any level that includes the Flood.
In order to see this glitch, play through the level until you see a Flood Combat Form, then approach until it starts a melee attack. As the arm swings back, shoot off the arm whip it would use. Although the arm whip is gone, you can still be damaged as if it were still there. This is a difficult trick to pull off, as the attack is usually quick, but with quick timing and good aim, you will be able to do it.
In Halo 3, lure a Combat Form to your position and get it to charge. Shoot off its whip arm and allow it to get close to you. It will melee you, and you will take damage even if the whip is gone. Then the Flood Combat Form will just follow you around.