Dead Man Hunting:

Turn on the Fly cheat and hold in R. The screen will go black.

Do not worry the screen will come back. But you'll notice you have no lives.If you turn off the flying cheat you'll die. So don't turn it off.

No enemies will notice you when you go back to the ground. Another thing you will notice is that there is no music. You won't have any weapons. You will be a zombie.