Duke Nukem 64

Play a Duke Match game with all items on. Kill another player or Duke Bot and enable "Invincibility" as soon as they face plant the ground. The player or Duke Bot will now be floating with their stomach facing the ground, resembling a UFO.

In the Rabid Transit level in single player mode, near the broken train is the blue keycard door. If you cannot find the card to open this door, use Duke's foot to kick it.

Play a multi-player game and choose the Castle Dukenstein level. This glitch is easier to perform when there is no Dukebot. Select two players. Have one player walk into the bathroom on the top floor and have the other walk into the room with the fireplace. Have the player in the fireplace room get the night vision on the couch and face the wall opposite the fireplace. Next, have the player in the bathroom walk up to the place where there should be a toilet. There is a chain-like thing hanging from the wall. When you walk up to it Duke should say "Where is it?". Pay attention to the player in the fireplace room. A wall should open up. Turn on your night vision to reveal the monster in the castle.
This trick requires two players. Play at the Castle Dukenstein level. Have one player get night vision. That player should move just outside the fireplace room in the hallway. Look at the wall directly next to the red curtain in the hallway outside of the fireplace room. Have the second player walk up the stairs in the castle. When Duke reaches the top, you should hear a monster growl. The wall should open, revealing another monster in the castle.
This trick requires two players. At the Castle Dukenstein level, have one person move back and forth outside in front of the castle doors. Have the other person move to the dining room area. Make sure they have their night vision on and look under the picture of the president. Make sure that the first player is still moving back and forth outside. If done correctly, you should see the third monster in the castle under the picture.

Make sure you have all of your guns. Since he cannot go off the stadium, walk on the edge of it so he walks in front of you. Make sure all you can see is his legs, then fire away. If done correctly, you should have a quick and trouble-free fight with out getting touched.

The Duke Burger Factory entrance is at the end of Gun Crazy. Step on the small box with the enormous shadow in the middle of the bend of the attic. Step up onto the box, and then turn around to see the hidden exit.

Enable the "Cheat mode" code and unlock the "Invincibility" and "All Items" cheats. Do not activate the "Invincibility" cheat yet. Start a Duke Match. Activate the "All Items" cheat. Choose the pipe bomb weapon and blow yourself up. As soon as you die, activate the "Invincibility" code. You will return to life. Disable the "Invincibility" code. You can now attack the Duke-bots, but they will see you as dead.

Locate the tunnel in the Freeway level. There is a large box on one side of the tunnel. Jump on top of the box, and from there jump on to the ledge. On the far side of the ledge there is a pair of night vision goggles. Jump a little bit above the goggles to enter the vents of the L.A.R.D. Police station. Use the night vision goggles, and follow the air ducts and shoot through the fan on the right side of the wall; or keep going and break the second fan that also on the right side of the wall. Duke may now explore the interior of the L.A.R.D. Police Station.

Continually jump while over hot spots to avoid damage.

Find the building across the street with the conveyor belts. If you go to the end, you will see a hydraulic press with yellow and black caution tape with a Cyberdyne Systems Model 101 endoskeleton that has been severed at the torso and crushed. When you walk over to it, Duke will say "Terminated". This is a reference to the 1984 sci-fi action movie The Terminator.

Go to the projection room in the level 1. Jump on the projector and get the Atomic Health. The wall next to you will open up and you can collect a grenade launcher.