Permanent invisibility

Permanent invisibility can be achieved on any level in Halo 2 where you play as the Master Chief. There are seven easy steps. 1) Get the Envy Skull.
2) Save and quit.
3) Start any level with MC.
4) Play the level until you get a checkpoint, then restart the level. (You keep doing this to have a good idea where/what triggers the checkpoint)
5) Once you know exactly when/where the checkpoint is, engage your Active Camo, then trigger it. Once you get it, revert to last to see if you are invisible at the checkpoint. If not, try again.
6) Once that is done, save and quit, and reset your Xbox.
7) Resume that same level and you will be invisible for the rest of the level! When you shoot or get shot, you will stay invisible, and even though enemies cannot see you, they may fire in your general direction so just be careful.

I am not sure if this carries on to consecutive levels so just try it yourself.