From Halopedia, the Halo wiki
The grenade slide is an animation error present in Halo: Combat Evolved. It causes a player's legs to stop animating as they throw a grenade. The glitch is only visible in Co-op or Multiplayer, as players cannot see their own legs in-game.
To see the glitch, two players (Person A and Person B) must cooperate.
The two players should find a room that they can run around in. Person A should obtain grenades (any type), while Person B should step back and watch Person A. Person A should start running, and as they run, they should throw a grenade. Person B will see Person A's legs stop moving, making A appear to slide along the ground. When the throw is finished, Person A's legs will return to normal.
When Bungie made the animation for throwing a grenade, they forgot to code the animation for the lower half of the player's body. So, when a Spartan model throws a grenade, its legs revert to a standing position. A similar glitch occurs when players throw grenades from the passenger seat of a Warthog or Scorpion, with the player standing up inside the vehicle to throw the grenade.