Continuity mistake: When Mary and Schaffer are firing the MP-40s from the back of the bus, a shot from outside of the bus shows the rear windshield broken/cracked one way but in the next shot from inside the bus looking out the rear windshield is broken/cracked differently. (02:15:35)
Revealing mistake: When the bus is being pursued, Smith steers it to trip several prepositioned explosives that blow up trees to block the road. In order to by pass a downed tree the Germans push a motorcycle around it, and you can plainly see that the tree was cut by a chainsaw. (02:18:40)
Continuity mistake: When Schaffer and Smith commandeer a motorcycle and sidecar, one shot shows the MP-40 slung over Schaffer's neck and laying across his lap, while in the next shot it is slung over left shoulder and hanging down his side. (01:01:40)
Revealing mistake: After Smith and Schaffer are captured and while they are being transported by car, Schaffer pushes one of the guards out of the car and a struggle begins which causes the car to crash. After the crash, the body of the guard that was pushed out of the car magically appears about 50 feet behind the accident (just before the crash there was no body). (00:49:25)
Continuity mistake: As Mary is starting the rappel down the rope out of the castle, her MP-40 is slung on her right side with the muzzle pointing forward, but in the next shot it is slung across her back with the muzzle pointing to her left. (01:55:55)
Answer: When the guard comes in, Burton explains what's going on. Also he is dressed as a German officer so his explanation and actions are plausible.