Assault on the Control RoomEdit
When a Pelican-riding cutscene starts to play, hold down the "throw grenade" button. When the cinematic ends, you will immediately throw a grenade and remain inside of the Pelican, even though map scripting would normally force you to exit. (In Assault on the Control Room, you can use an alternate method; button-mash X (action button) or E (for Halo PC) to re-enter the Pelican as soon as you exit.) At this point, the Pelican will fly away on its scripted path, with you still inside. You may exit at any time; if you do not, the Pelican will eventually strike a kill barrier, resulting in your death.
343 Guilty SparkEdit
Immediately after you take control of Master Chief, throw a plasma grenade. This should allow you to stay onboard and gives you the opportunity to witness the Unarmed Marine Easter Egg and the Floating Assault rifles glitch.
In Halo, through modding, it is possible to board a pelican alongside with fellow human survivors. However, after a while, the pelican will either crash or "drop" the player and the passangers, sometimes killing everyone.