The Floodgate Vacation is a glitch that allows you to get out of the normal game boundaries on the Halo 3 campaign level, Floodgate.
Vacation on top of the Flood ship
Play the level until you get to a part where the Shadow of Intent sends down two friendly Phantoms to drop off Elites. You will see three shipping containers (single and double boxes) stacked up like a staircase. Kill the nearby Flood forces and pick up a Deployable Cover.
Go back to the containers and push a crate next to the lowest shipping container; climb that container by jumping from the crate onto it. Deploy your cover on this shipping container, so that you can use it to get to the upper shipping container; however, don't climb to the top just yet. Advance until you can find another Deployable Cover, and then go back to the containers. Climb up the containers, jump onto the slanted part of the building in front of you, and walk left until you get to some reddish-brown pipes coming out of a "ramp" on the roof. Climb up the "ramp" until you can get up to the slanted top of the roof.
Walk diagonally along the invisible wall toward the crashed Flood ship. Don't be afraid to walk over the invisible areas -- there is solid ground there, though you cannot see it. When you get to the place where the building you are on connects to a wall, walk to the edge of the roof. Turn around, deploy the Deployable Cover, and use it to climb onto the roof with the balcony.
Now walk towards the crashed ship. When you get to the part where the roof is smashed by the ship, jump onto the latticeworks of the ship and then onto the ship itself. Now, you can climb all over the Flood ship. (Be careful not to fall down to the place where the Arbiter sends you into the ship to retrieve Cortana, as a "one-way" invisible wall will prevent you from going back up.)
Vacation behind the fence
Start from the beginning of the level and play until you get to the area with the fence and the generators. Jump onto one of the concrete barriers and then Crouch Jump onto the generators. From there, Ledge Jump and Crouch Jump over to the top of the fence. It will take a while, but it is possible. Once over the fence, you will be in falling pose. Keep moving forward and you will eventually fall. Not much is out here, except for a few dead Marines and a dead Flood body, and some ammo. There is however, a random equipment case in one of the silos that is just there for no apparent reason. Also, there is no way back inside the level from this vacation.