From Halopedia, the Halo wiki
This glitch occurs on the Cairo Station level. After the first cutscene, as you descend 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 cover. Melee the crate with his cover on it repeatedly. The crate will eventually slip from under the cover, but the cover will remain motionless in the same position. It cannot be moved; it is just a minor programming error by Bungie.
This glitch can be done on Quarantine Zone (level). Several methods can be used to witness this glitch, the first being to simply walk into the Index Chamber normally and stay on the far left of the wall. You will pass architectural outcroppings to your left, and several small structures in the middle of the area to your right. Slowly advance until you reach the area just before the 'Human' gondola entrance, after passing several lifeless Elite Combat Forms. Note that you should still be relatively close to the wall separating the actual chamber area from the area you are currently in. Activate your zoom, preferably the default zoom, as the lack of a reticule makes it easier to see. You should see at least one SMG floating in midair under an architectural outcropping similar to one you passed beneath on your way over. Due to the rendering limitations of Halo 2, the SMGs can only be seen while zoomed in, as they will not be rendered when zoomed out (you are too far away). Moving closer will cause the SMGs to disappear.
Another method requires the Sputnik skull. 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, but you have not yet entered the Index Chamber. 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.
Much like the floating Assault Rifles glitch from Halo: Combat Evolved, this glitch is actually intended, though the fact that the player is able to see them was not. The SMGs were left floating to imply that some Marines survived the Gondola ride and were in the process of fending off the Flood. Had Bungie elected to use Marines wielding the SMGs instead of leaving them in midair, the AI characteristics would have caused the Marines to move around and communicate, thus alerting the player to their presence. The fact that the SMGs float adds to the mystery of the level.
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 antenna. 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.