CTF Weapons Glitch
From Halopedia, the Halo wiki
So the player has his Assault Rifle out, and is holding the flag. He trades the flag for a Needler, and drops his Assault Rifle. This is because parts of the game didn't check if he was holding a flag, so the game swapped his Assault Rifle for a Needler.
The player then picks the flag back up, and the dropped Assault Rifle vanishes. This is because the player was picking the Rifle back up; however, some flaw in the process prevented the game from forcing the player to drop their Needler, leaving them with three weapons.
Note: This is one of many Three Weapons glitches.