Continuity mistake: When the team are about to jump from the cable car into the river, the view from ground level shows a bare road with no snow on it on the right and a line of trees on the left with German trucks parked next to them, but the view from the cable car shows a snow-covered road and no trucks beside the trees. (02:06:00)
Continuity mistake: Major Smith forces Carnaby to reveal his true identity by firing a single bullet which fragments the upper section of his chair. Soon after when Smith stands behind Carnaby (revealed as Cartwright Jones) the chair is undamaged.
Continuity mistake: When Smith is driving the bus, there are at one point several bullet holes in the bus' windscreen and in the next shot they have vanished.
Continuity mistake: In the scene where the Germans are beating down the door to the hallway leading to the cable car, when the door is about to give way, in the clip from inside the door, it is moving a lot under each blow, but when seen from the outside it doesn't move at all.
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)
Continuity mistake: When the team is about to jump from the cable car into the river, seen from the cable car, they wait until they get close to the shore. But when they jump, seen from below, the cable car is moving across the river from the far side, and they land in the middle.
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)
Continuity mistake: Smith puts on one glove just before he hops onto the cable car with the two German spies. The next shot shows him with two gloves on.
Continuity mistake: The car that Schaffer and Smith are riding in with the Germans is launched in the air by hitting a pile of snow and then comes to a sudden stop by hitting a pile of gravel where the Germans are killed. What happened to the pile of snow when Schaffer and Smith push the car backwards?
Continuity mistake: After Smith and Schaeffer are done rigging the trees and poles with explosives, Schaeffer gets off the motorcycle and you can see the seat of his pants is wet. As they are entering the shed, the seat of his pants is now dry.
Continuity mistake: When Smith and Schaffer intrude on Carnaby's interrogation, the Gestapo officer and Mary are having drinks in the café, where he makes a statement that might reveal a fault in her identity (a location in Dusseldorf). Her hand is fixed on her chin, but the next cut shows her hand fully on her cheek.
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)
Continuity mistake: Major Smith (Richard Burton) forces one of the traitors out of the castle window on a rope. He is wearing standard battle dress. When the traitor is shot by one of the Germans, he falls wearing a camouflage smock.
Continuity mistake: In the scene where the group parachutes in, the sky is cloudy and we can see snowfall, but watch carefully, as the men make their way to the equipment containers and then to find Sgt Harod we constantly see shots switching between cloudy and snowfall, then to no snowfall and sometimes sunlight, then back to cloudy and snowfall again. This switch back and forth is even more evident on the closeups of Richard Burton.
Continuity mistake: In the scene where they all jump from the sky tram into the water after escaping from the castle, we see the blonde woman (Mary) swim to shore. As she gets out, her hair is absolutely soaked and matted to her head. When the angle of the camera changes, we see her only this time, he hair is almost perfect and only slightly damp and is actually blowing in the soft breeze.
Continuity mistake: When Burton and Eastwood jump from the cable car into the river, Burton jumps before Eastwood. However, the next shot downward has the Eastwood stuntman landing in the river first with the Burton stuntman still above him. We know it's Burton because of the bandaged hand. (02:06:15)
Continuity mistake: When Mary Ure goes to secure the rope over the wooden beam to allow Smith and Schaffer to climb into the castle, she starts by hitting a narrow piece of wood which comes loose. The scene then momentarily cuts to the guys on the cable car. When it cuts back to Mary, the narrow piece of wood is back in place and she is knocking out the much wider piece next to it. (01:05:15)
Continuity mistake: When Burton and the others go up the wooden staircase to approach the radio room, an upright beam is already charred by fire, but this wood only catches fire later when Eastwood throws the first grenade back at the Germans.
Continuity mistake: When Burton kicks Donald Houston off the cable car, they have been fighting on the side of the cable car. However, the next shot of Houston falling shows him falling from the front of it.
Continuity mistake: When Smith is rapelling out of the castle and nearing the bottom, Jones/Carnaby is to the left of Smith but in the next shot he is to the right of Smith. (01:56:50)
Chosen answer: This is not unusual in older films. Earlier costume designers were less attentive to historical accuracy and freely incorporated current fashion trends into period movies. It was just an accepted practice and movie audiences back then were often less discerning and/or unaware of inaccuracies. Today's costumers have greater access to historical information, do more research, and strive for authenticity.
raywest ★