Kelly's Heroes - Eternal Ammo Supply
Towards end of the movie when Kelly's group is in the town of Nancy and the Germans now know they are there, OddBall's Sherman tank has a German Tiger Tank trapped in a narrow alley. When the German cannot rotate his turret, the German commander comes up out of his hatch to see what's wrong.
Kelly, (Clint Eastwood) who in on the back of OddBall's Sherman tank, behind the turret, grabs the 50 cal machine gun and cocks it to fire.
At this point, watch the "open frame" ammo box. You can clearly see the folds of the belt of ammo stacked within it. As Kelly fires the machine gun, you can see the belt being used up as it leaves the box and feeds through the machine gun.
When he finishes firing the weapon and lets go of it, the ammo box is mysteriously full again.
The problem is that there is no ammo box...the .50 cal ammo is folded on the metal frame that holds the ammo box.