Two Spartan Lasers on Halo

The Two Spartan Lasers glitch in the Halo 3 Campaign level, Halo, is a glitch that makes it possible to carry two Spartan Lasers at once (not to be confused with dual wielding).

Traits
The second Spartan Laser, used by Sergeant Johnson, is programmed as a completely different weapon, which is why a player can carry it along with another, normal, Spartan Laser. The second Spartan Laser is equipped with infinite charge. The reasons for these traits are unknown -- NPCs never run out of ammunition regardless of what weapon they use, so it is unclear why a special variant of the weapon with infinite charge would even be necessary. It seems likely that Bungie wanted to prevent the player from depleting the battery of the Spartan Laser acquired from Johnson, otherwise a player might find himself unable to defeat Guilty Spark.

Outdoor Laser
First, proceed up the tower with a Rocket Launcher. When you reach the top, do not kill all of the Flood forces, or Johnson will come down and the glitch will not work. Instead, run to his position and blast him off the cliff with the Rocket Launcher or a Plasma Grenade. Once he falls off the cliff, take his dropped Spartan Laser. When he gets back up, he will grab the weapon you exchanged for the laser.

At this point, kill Johnson again. When he gets up, he will inexplicably carry a new Spartan Laser. Kill him a third time, by meleeing him to death. Then, trade your non-laser weapon for his dropped Spartan Laser. It is this laser that possesses infinite charge.

To check if you've done it properly, repeatedly press the Y button to check your weapons.

Cortana Laser
Start Cortana from the beginning, on Normal or higher. Find Cortana, and leave that room. Then, while in the room where you destroy the generators, try to jump on a ledge jutting out that is about middle height. After a few tries, (grenade jumping and the Cowbell Skull may be necessary) you should be on the slanted ledge. When you're done, look around the slanted ledge, and eventually you should find a pile of bones near the edge. Among those bones is the Unknown Skull. Take that skull with you until the end of the level, and in the next level, you should have a Spartan Laser as a starting weapon. You can kill Johnson, or you could just take the Spartan Laser he gives you in the battle against 343 Guilty Spark.

Control Room Laser
Another method involves killing Johnson before he gets to the door of the Control Room; melee him three times to do so. Take his Laser. During the battle in the Control Room against 343 Guilty Spark, at the part when he gives you his Spartan Laser, simply swap your non-Laser for it. However, be careful; the second Spartan Laser does not have infinite ammo and usually has only a few ammo points remaining. It is also possible to do this by taking Arbiter's Enery Sword and meleeing Johnson with the sword. If done correctly, the Spartan Laser he holds will fly far away from him. Exchange your weapon for this and wait for Johnson to get up. He should have a new Spartan Laser instead of picking up your weapon. Now kill Johnson again (If necessary, kill Arbiter first so he doesn't kill you) and take the second Spartan Laser. This has infinite charge.

Trivia

 * Interestingly, due to the fact that weapons left outside the Control Room's first entrance door remain there, it is possible to leave a Spartan Laser behind to collect after the battle with 343 Guilty Spark and use it during the latter part of Halo.
 * The first Laser that Johnson drops generally has an odd amount of charge; this is true of many NPC-carried items, including the Arbiter's Energy Sword. It is Johnson's second laser that has infinite charge.