Riding a Pelican
From Halopedia, the Halo wiki
On some levels in the Halo trilogy, it is possible for a player to climb or jump on top of a Pelican and ride the ship to its destination -- Riding a Pelican. The glitch has been seen, so far, in Halo: Combat Evolved, Halo 2, Halo 3, and Halo: Reach.
The glitch nearly always has the same results: the Pelican will either fly players outside of the map and directly into a kill barrier; or it will fly outside the map, fall or sink (sometimes violently), and drop players directly into a kill barrier. This is due to the AI Driver Despawning. The Pelican is meant to fly away and despawn. The pelican cannot despawn with the player in it, causing the crazy/bumpy ride then crashing!
Halo: Combat Evolved
343 Guilty Spark
At the beginning of 343 Guilty Spark, when you are still in the Pelican for a few seconds, throw a Frag Grenade out of the Pelican; this will cause you to stay inside the Pelican. It will fly around the map, eventually going into the hills and hovering there for the remainder of the level. The hills span around the perimeter of the map and if you go around them, you will eventually find an unarmed Marine who stands completely still.
It is also possible to jump onto the top of the Pelican using a boulder at the nose of the ship. After that, you must time your jump right to get to the ledge that was to your right at the beginning of the level. Using this method, you can explore the area on foot.
Assault On The Control Room
At the beginning of Assault on the Control Room, Pelican Echo 419 will drop you off in a Forerunner building. As soon as you are dropped off, it will say "Hold X to ride Pelican"; follow the instruction. If done successfully, you will be able to ride it for a short time. At the end of the ride, you will be booted out, and die. However, if one aims at the Pelican once they are booted out, they will see "Hold X to flip Pelican". If they do so, the pelican will disappear and they will be pushed onto a ledge, near which they can activate the Siege of Madrigal Easter egg.
It is, however, possible to survive if you are playing Co-op mode and one person exits the Pelican at the right spot. When you get in the Pelican at the start of the level, you will always pass through a checkpoint. The checkpoint is always there, regardless of whether you get in or not. If one would wish to continue the level after the ride, you'd have to hold X to get out again.
This can also be done on the first bridge that you come to. A Pelican carrying Fire Team Zulu will begin its descent near the middle of the bridge. If you jump at the right time, you will be able to land on it, hop off at its descent, and skip a very aggravating part of the level on Heroic and Legendary.
Death By A Pelican
When the level starts, if the player continues to rapidly press the enter/exit vehicle button (or action command for PC players), the player will hop out of the Pelican and then hop back in. After a few moments, the Pelican's script will kick in, and it will fly back in to the shaft room it came out of, leaving the two Sangheili standing there, firing at you in vain. Given the size of the shaft room, Foehammer will have to go vertical, nosing over roughly 80-85 degrees below the horizontal. As the player looks out the rear of the Pelican, the depths of the shaft room become visible. Eventually, Foehammer will travel far enough down that the view becomes obscured by a turquoise fog. After a few seconds, the player will mysteriously be ejected from the Pelican and die almost instantly when the Pelican hits the third of three large spinning disks approximately halfway down the shaft.
If you jump onto the rooftops at the beginning of Outskirts and completely ignore the battle in the first part, a checkpoint will appear and Sergeant Johnson will spawn and begin boarding a Pelican which will also come. If you jump onto the Pelican, it will fly off with you on it. You won't get far though because it disappears in midair.
Play through Metropolis until you get to the part where you have to rescue some Marines trapped in a building. You will be on a street with a couple of Jackal snipers, two Type-26 Wraiths, and a couple of Type-32 Ghosts. Another Wraith and two Phantoms will come momentarily, followed by two Pelicans.
Go to the far staircase between the streets; this is the one closer to the building the Marines were in. This is the point where the Pelican will drop off some more Marines for you. You need to be the top of this staircase before the Pelican begins its descent. As soon as the Marines have exited the Pelican, jump toward the passenger bay and hold the X button. Aim for the bottom of the Pelican, right near the back entrance. An easier way to do this is to park a vehicle where the Marines are dropped off, climb on it, and wait for the Pelican. This should allow you to climb in the cargo bay instead of boarding.
After a short trip, the Pelican falls to the bottom of the level. Along the way, a blue notice like the one that tells you to get to cover will pop up saying "you must be in a DRIVER seat to drive a vehicle". In Halo: Combat Evolved, a free-falling Pelican would travel the exact same path every time, but things might be different here. Occasionally, on Co-op, everybody will land on a giant concrete platform (too far down to get back into the level, and too far up to continue to the very bottom). The Pelican may continue its way down, while players and Marines are stuck there. Sometimes, instead of landing on the giant concrete platform, players land on an invisible platform beneath the giant concrete platform. Also, on co-op, when both players enter into the pelican, the second player is standing on the seat, with their head sticking out through the top of the pelican and their arms in the style of a cross.
Another example is on the level Delta Halo, where you continue as normal until the Pelican comes in and drops off the Warthog. Get to where the Shade turret on a small hill was, and then wait for the Pelican to come to a halt. Then Explosive Jump up to the Pelican. It will fly around with you on its back for a little while, and then accelerate, causing you to fall off of it and to your death.
Another way is to continue until the part of the temple where the Pelican starts to drop off weapon pods. You can jump atop the temple by crouch jumping and grenade jumping; from there, you can jump normally onto the Pelican. It will fly off into a hill and disappear. You will be very far from where you are supposed to be, and you may need to revert to the last checkpoint as if you tried to return to your previous position without reverting, you would most likely fail.
While you must ride the Pelican at the end of Sierra 117 to complete the mission, you can instead get into its troop bay and kill the Marine on the turret. You can then take control of the turret, but once you get on the turret, the last cut-scene will begin.
The Pelican that shows up in Crow's Nest's first hangar can be boarded. After you clear the hangar, jump on Deployable Covers while the Pelican is coming out of its spot. If you time it right, you can get into the Pelican's bay (or, if you are good, possibly it's cockpit while it stops being part of the scenery and an actual vehicle). It may take a couple of tries, and could be a little frustrating.
Another Pelican appears when you take out the Jump Pack Brutes. You can jump on the Pelican. However, as soon as it takes off, you will die.
At Rally Point Alpha in The Storm, right before the Scarab fight, a Pelican will enter the lakebed and drop off two Mongooses. The door to the troop bay will open, and both vehicles will hastily drive out. There will be a short moment in which you can jump into the Pelican right after the second Mongoose pulls out. You will stay in for about two seconds before it closes again. Then, you will ride it for about two to five seconds before inexplicably dying.
This glitch can be expanded, and if done quick enough you can get a checkpoint while inside the Pelican. This glitch is easy when done if loading Rally Point Alpha.
It may take one or two attempts, but find out exactly where the Pelican stops descending. Just when it stops, jump at the door; it is not solid, so you will go through it and into the Pelican. Jump immediately or you will get splattered or forced out of the Pelican by the first Mongoose; you must crouch jump slightly so you don't fly out, land on the mongoose, or die. If done right, you'll take some damage, but you'll be alive.
Quickly get to the second Mongoose, and force the Marine out so you can drive it; the Marine might get stuck or end up in front of the Mongoose. You have about two seconds before the Mongoose will get forced out; driving the Mongoose, however, will prevent it from being forced out. You will still die along with the Marines inside and the mongoose if you stay inside. You can drive outside the pelican when the doors are closed, as they are not solid.
If you drive out properly and at the right moment, the Marine that was originally driving the Mongoose may be standing inside (as in, overlapping) the non-solid door or right next to it (such that he appears to be hovering in the air). If you continue to watch him, you'll find that he will experience a similar fate to your own when riding the Pelican: he will either fall out of the Pelican (and probably off the cliff), or he will suddenly drop dead when it hits a kill barrier.
Starting at Rally Point Bravo on The Ark, after you have killed all the enemies downstairs, get to the extraction point before the Pelican arrives; this will prevent the cutscene from taking place. When the Pelican comes near to you, jump at the Pelican's troop bay and hold RB when you see the message Hold RB to ride in Pelican. If done correctly, you will be riding in the Pelican and the Pelican will not take off and no cutscene will take place. You cannot do anything but shoot while riding in the Pelican. If you get out, you will fall to your death.
At the beginning of The Covenant, when you destroy the Anti-Air Wraith, a Pelican will drop down to deliver a Mongoose; this is your chance to jump inside the Pelican. When you do, it will stay floating for a while until it finally takes off. It will take you around to the area where you first started, and then an invisible wall will push you out. It's easier in Co-op.
When you return to the beach after deactivating the first tower, another Pelican will appear, along with two Hornets. You can use a Hornet to enter the Pelican. Fly up to the Pelican, and jam one of the Hornet's wings inside of the Pelican by flying up and down multiple times. You need to jam the wing on the side that your character climbs out of the Hornet. Then, simply hop out.
Alternatively, you can have one player stand on the wing of a Hornet, while the other flies near the Pelican's troop bay. After boarding the Pelican, the player may be forced out; they can avoid this by standing on the side that the Marine on the turret is facing, as the marine will block the player's movement.
If done correctly, your character will jump out of the Hornet and into the Pelican. However, nothing will actually happen until you get out and continue the level unless you do the glitch on Co-op, and have another player continue the level. If the player in the Hornet flies over safe land and is far enough from the player in the Pelican, the latter will be teleported.
The Pelican that drops off the Scorpion is also rideable. If in Co-op, one person (Person A) boards the Pelican and the other (Person B) takes off in a vehicle, then an interesting glitch can occur. After Person B travels far enough, Person A and the Pelican they're riding will teleport to Person B's location. The Pelican will somehow drop through the walls of the map, though it will not go through the Scorpion. Note that it can also be flipped.
During the battle against the two Scarabs, Miranda Keyes' Pelican—seen flying during the battle—is also rideable. When Sergeant Stacker yells, "Hornets inbound!", if you stand on the little ledge where the Hornets land, you can see where the Pelican lands for a couple of seconds and then flies off. Restart from a checkpoint, and rush to that position (it is recommended that you use a Warthog). The Pelican's troop bay will be closed, but if you jump onto the hood of your Warthog, and jump while holding RB, you will enter the Pelican and will be able to actually ride in it. Another method is to get into a hornet and jump out of the driver's seat when close enough. If done correctly, and RB is hit on time, you'll be able to get into the pelican. Alternatively, in Co-op, one person can stand on the wing of a Hornet, while another player hovers to the Pelican.
ONI: Sword Base
There are 3 Pelicans in this mission, the first and easiest one appears outside the base and drops off a Warthog. If you use the technique mentioned above you should be able to escape the map and walk around the area behind Sword Base, eventually, if you keep walking you'll start going up and hit an invisible wall. The other two Pelicans will simply drop you back on the map.
Tip of the Spear
There is one Pelican in this mission that comes close enough to the player to board, it appears after destroying the second Tyrant to drop off troops. If you Armor lock on it, it should take you out of the map and you can walk around in the battle below the ridge.
Long Night of Solace
There is one Pelican that comes after clearing the beach to drop off troops if you Armor lock on it it takes you to the back of the launch facility where you can walk around the building and roads behind it.
In Halo 4 you can drive a Pelican in the level shutdown, but you can also drive it in moments where it's not meant to be driven. You can ride in the back seat of it if you are playing Co-Op.
In Halo 2, Pelicans' flight patterns are scripted. When they are not scripted, Pelicans often behave erratically and exhibit bizarre physics phenomena when interacting and colliding with level geometry. An invisible wall separates their troop bays and cockpits, a trait not seen in Halo 3. Furthermore, Pelicans can be flipped, but the displayed message misidentifies them: "Press X to flip Banshee." Flipping a Pelican results in additional physics oddities including a rapid and uncontrollable rotation.
Killing or ejecting a Pelican's driver will cause the unmanned Pelican, its riders, and its turret gunner to completely fall through the level's terrain and to their deaths. A common method is to throw a grenade (typically a Plasma Grenade) through the (non-solid) door that separates the troop bay and the cockpit. It is also possible to throw a well-aimed Plasma Grenade at the front of the Pelican. The resulting blast can kill the driver, causing everything in it to die, like the other method.
Pelicans can be flipped, though the "Hold RB to flip..." message does not show.