Floating objects

Floating Objects are objects that hover in the air. In Halo 2 's campaign, there are at least two such objects.

Floating Hat
This happens on the Cairo Station level. After the first cutscene, as you descend on the stairwell, you will turn and there will be several SMGs and Battle Rifles. Adjacent to this, is a crate where Sergeant Major Avery Johnson has left his cap. Melee the crate with his cap on it repeatedly. The crate will eventually slip from under the hat, and the hat will be there, motionless. You can't do anything else with it. It's just a minor programming error by Bungie.

Floating SMGs
This glitch can be done on Quarantine Zone (level). Sputnik skull required. Proceed through the level like you normally would, until you reach the Gondola. At the end of the ride, get off and walk to the right until you reach the wall. Grenade jump on the wall and jump off at the other side. Now you should be in front of the Gondola Sergeant Johnson and Miranda used. Head towards the Index, but before you go through the small corridor where the cutscene starts, there will be two floating SMGs shooting at empty space. You can pick the SMGs up, and they will keep firing; you can be wounded by the bullets. They have 60 bullets each in them. Even if you drop them they will keep firing. Notably, Miranda Keyes used SMGs on the Arbiter during a cutscene. The SMGs in this glitch may be the very same SMGs.

Floating Ammunition
This glitch can be performed on the level Outskirts. Play through the level until the part just before the tunnel. There is a structure before you enter the tunnel with a rocket launcher on it. Go to the top of this structure and look for the thing that looks like a very tall antennae. There is a box right next to it with ammunition on it. Melee the box away and the ammunition will stay there, floating in the air. You can still use the ammo if you need it.