Time travel glitch
From Halopedia, the Halo wiki
The time travel glitch is a glitch present in Halo 3's Campaign levels The Covenant and The Ark. The glitch allows players to "travel back in time", going back from the Scarab battle to the third tower; it can also be done in reverse on The Covenant.
The glitch earned its name from an incident where the first (known) person to discover it witnessed what appeared to be a Brute floating without the aid of a jump pack; the player jokingly reasoned that "time has passed very fast to the point where Brutes are so advanced they can float with out jetpacks [sic]". In reality, the game had simply "believed" that the Brute was standing on a Deployable Lookout Tower; as the player approached the Brute, the game corrected its error, causing the Brute to fall to the ground.
Basically, a player has to go to the Scarab Tank battle and find a way to glitch themselves over the mountainside to the left of or behind the second tower. From there, they need to move into the void. After traveling into the void for a little while, they will find themselves "warped" back to the third tower's exterior.
The glitch can also be done in reverse. If a player manages to glitch themselves onto a mountaintop at any outdoor point before they ascend the icy slope next to the third tower (said slope leading to the Scarab Tank battle), they can travel "forward" in time by simply crossing over into the Scarab Tank area.
Instructions (going back)
One method involves driving a Qavardu Workshop Prowler or a Type-32 Ghost up a certain cliff in the Scarab Tank area, glitching it over the wall and into the void. Once the player has reached the edge of the void, they must slowly back into the void. At some point, the player will be teleported on top of the cliffs in front of the towers. From there, they may enter any of the three towers, except the second tower, which can only be entered through another variation of the Time Travel Glitch.
Instructions (going forward)
In Co-op, on the way to the first tower, you or your companion should grab a Type-32 Ghost. Clear out the enemies at the first tower's entrance, and then go up the hill that leads to the Shade. Have one player (Person A) pilot the Ghost, and the other (Person B) stand on top. Person A should ease the Ghost against the cliff wall, and the use the boosters to propel it to the ledge of the tower. Person B should jump when closest to the tower. If done right, Person B will land on top.
Person B should then travel to the next ledge and Explosive Jump up. Person A should kill themselves so that they spawn near Person B. The two players should then continue to ascend the cliff. When they get to the point where they cannot go any farther, they should jump to the right and land on an invisible wall. The players will be in their jumping pose, but they will still be able to float their way up the snowy cliff. Keep going and the game will freeze temporarily; after the delay, the duo should be teleported to the other side of the barrier: the Scarab Tank area.
Alternatively, a player can hijack the Wraith in the valley, park it on top of the destroyed Shade, get out, and then grenade jump from it onto the cliff. At this point, the player would continue as they would in the previous method after grenade jumping. Upon reaching a mirror-like graphical glitch, the player would go to the corner of the tower where the snow first meets the tower (away from the mirror). From there, they would walk into the little crack between the snow and the tower. At that point, the game will freeze temporarily, and after the delay, the player will find themselves behind the tower, able to explore the area but unable to return to the other side.
Bear in mind that additional kill barriers may be present.
Instructions (forward and back)
It is possible to explore the exterior of the level using a Hornet. To do so, have one player (Person A) gather a Plasma Pistol or a Power Drain. Person A should then "go forward in time" using a Ghost, as described above. At least one other player (Person B) should continue the level as normal, right up to the point where the Scarab Tanks land.
At this point, Person A should stay on the mountainside, keeping a watch out. The Hornets that are deployed into the area as reinforcements are flown in from Person A's position. When Person A spots a Hornet, they should immediately stop it by deploying their Power Drain or firing an overcharged Plasma Pistol shot at it. They should then hop into the vehicle, and travel back in time by flying over the cliff and into the void.
From there, Person A may freely explore the seaside portion of the level.
Instructions (The Ark)
In The Ark, the glitch can be done in Co-op, at the area where you battle the Scarab Tank. Both players must have a Brute Chopper and a Grav Lift. The players will need to use the Choppers to drive up the rocks on the right side of the map. From there, they can bypass the level boundaries. Once outside the level, they need to go to the other side of the map and climb up the long ramp to the top, where they will find a wall. There, they should use the Grav Lifts to scale the wall. At that point, one of them should walk forward to complete the glitch.