Other mistake: In the scene where Burton and Eastwood are in the radio room, two grenades are thrown, which blow the main radio room door off the hinges and Burton and Eastwood run behind the small door and hide in that room. The German soldiers then run up and one of them throws another grenade supposedly into the radio room. However, the actor in his attempt to throw the grenade into the room instead throws the grenade into the door and it bounces back into the hallway in which he is standing and lands in front of him, yet no one reacts to this and seconds later we see an explosion as if the grenade had actually gone into the room, but it is really on the floor in front of the door and would have certainly blown the German soldiers to bits.
Other mistake: When Mary first meets Major Smith in the shed on the alpine pasture she complains that she ‘nearly froze to death in that damn plane' yet when she emerged from her hiding place on the aircraft she was only just putting her gloves on. If she was that cold then she would have had the gloves on already. (00:09:05)
Other mistake: In the scene where Smith and Schaffer are riding with the SS in the staff car, after the car hits the snow bank, Smith picks up the SS Colonel and puts him back in the car. The Colonel is wearing a Major's collar tabs.
Other mistake: When Richard Burton is removing the light bulbs to darken the cable car as it approaches the German gunnery team, he screws them clockwise instead of counterclockwise. He is never shown actually removing the bulbs nor do they darken as he is "loosening" them.
Other mistake: When Smith and Schaffer are preparing the rope to enable them to climb into the castle, their footprints on the ‘snow' are more consistent with footprints on white sand. (01:08:35)
Other mistake: When escaping down the walls by abseiling, the actors hold rope with two hands - impossible to do down such a height with small width of rope. The stunt folk have rope going round their backs as they come down, abseiling properly.
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.