These from Donald Fournier:

• In any level where the Energy Sword is, grab it then look at your shadow (you have to be on a map where you can see your shadow, or else it will not work). Put it away by taking out another weapon, then pull out your Energy Sword while looking at the ground. You should see his hand move and pull out the handle, but, when he turns it on the blade doesn’t show up in your shadow.
• On Zanzibar, the sun is invisible in the Sniper Rifle’s scope; it sees what you see but when you point it at the sun it doesn’t show up on the scope.
• While in the passenger side of the Warthog with any weapon, press B to melee. The Chief will melee, but the action is not seen.