Hitchhiking Flood
From Halopedia, the Halo wiki
To see a Hitchhiking Flood, you must play the level Quarantine Zone on Halo 2. Get a Ghost and drive it through the level until you get to the big door (just before the "elevator" part of the level) that two Flood Combat Forms come out of, ram one back into where it came from and let it board your Ghost. If the door closed behind you with a little luck the Combat Form will 'freeze' in a way. The Combat Form will stay on your Ghost in attacking position, but it won't attack you, and you can drive around with it on your Ghost.
If you exit the Ghost, the Combat Form will stay on the Ghost and you can't get back into it, unless you kill the Combat Form.
