Enter a Scarab Glitch
From Halopedia, the Halo wiki
The Enter a Scarab Glitch is a glitch in which you can get on board the Scarab before the cutscene "Backseat Driver" in the last level of Halo 2, The Great Journey. Although you can get inside it, you cannot operate it in any way at all.
There are a total of five methods to get inside a Scarab. These methods are:
When you get to the platform, Grenade Jump to one of its legs. Then Crouch Jump onto the deck. You can now go inside the Scarab (there is no shield), and push the invincible Johnson to the edge of the Scarab, and get him to fall off the Scarab's deck, and down to the sand far below.
Before Johnson jumps aboard it for the first time, you must bunny hop to the edge of the platform. When you get there, jump. It is extremely important that you time this perfectly. As the Scarab moves back a bit, jump right then. You should end up going higher than usual. This can take a few attempts. If it works, you will jump towards either the left or right leg. If done correctly, you will land on the leg. Then crouch jump onto the deck.
Take a Ghost throughout the entire level and bring it with you to the platform with the Scarab. Boost at the Scarab, and stop right before the edge. Jump on top of the Ghost and then up to the deck of the Scarab.
Note: Pushing Johnson over the edge will stop the cutscene. Do not attempt this unless you want to revert to the previous checkpoint, or have already beaten the game.
First you need to activate the Sputnik skull and then go to the part of the level where two Wraiths appear to keep you out of the structure. To your right is the rock that hid the closer Wraith from view. You need to grenade jump onto that rock, and from there grenade jump onto the platform.
When the Scarab lowers, jump before the cutscene starts and you will not see a purple wall. To achieve this you must not kill any enemies when you reach to see a Mgalekgolo.