Floating Plasma Grenade

Floating Plasma Grenades can be seen in Assault on the Control Room, when you get to the first lift, stand behind the sign that says 'Press X to Activate'. Lift and throw a plasma grenade into the center of the lift. The lift will go down, but the grenade will float in front of you. The grenade will harm you, so if you have low health, you will die after it explodes.

Elevator Grenade
A glitch involving grenades can be observed when you throw a plasma grenade into the air while riding an elevator. Instead of exploding, the grenade will keep bouncing until the elevator stops.

Floating Grenade
On Pillar of Autumn, after Captain Keyes gives you the pistol, kill the Grunts and return to the bridge. Kill one of the crewman or Captain Keyes and the invincible Marines will enter the bridge. Let them attack you until you have low health and throw a plasma grenade WHEN, not before, you die. The grenade shouldn't leave your hand when you die. The grenade will float in the air for a while and explode.