Revealing mistake: When the tank/truck driven by Marvin is crossing the stone bridge at the end, they are met by a small German gun vehicle, which they run off of the bridge. The camera angle changes to below the bridge and you see the German vehicle fall from the side of the bridge into the water below. If you look at the bridge material that shatters you can see pieces of wood and hollow areas where obviously it's not real stone. (02:25:00 - 02:30:00)
Revealing mistake: When the 25 pounders fire, there is no recoil.
