Revealing mistake: When the cable car is seen in long shot coming down from the hill (carrying the film's heroes) it is painfully obvious that while the cable car is real enough, the mountain behind it is fake.
Continuity mistake: When Smith fires at the last German Kübelwagen chasing his team on the Oberhausen airfield runway, the scene alternates between two perspectives of Smith firing from the bus's engine cover: One from the front, the other from his backside. In each of these alternating shots, Smith holds the front part of his MP40 in a different grip. (02:20:55)
Continuity mistake: After the explosions start in the castle, three SS soldiers knock on the door of the room where the General, Colonel Kramer and the guard are lying dead on the floor. The General can be identified by the red stripe on his trousers and the guard is on the right facing away from the camera but the body in the centre that is supposed to be Colonel Kramer looks nothing like him. The face is different, and he has swept-back blond hair, whereas Kramer's was much shorter and greyer. It's a very short scene, so they probably got members of the film crew to pose on the floor rather than the actors themselves. (01:52:31)
Revealing mistake: The team parachuted onto a mountain top while it has been snowing for some time, but when they go looking for Harris the missing radio operator, you can see footprints in the snow.
Character mistake: Lt Schaffer (Clint Eastwood) is wearing his War Merit Cross With Swords badge upside down on his tunic. It's supposed to be worn with the sword handles at the bottom but his are at the top. Best view of this is when he is leaning against the bar of the tavern at 00:36:03 and has just watched Major Smith get slapped by Heidi. (00:36:03)
Continuity mistake: It may not be a mistake but early on in the film when Smith and company are preparing to jump from the aircraft, the jump master removes the fuselage door completely and sets it aside. The same aircraft returns at the end of the film to collect them but when Colonel Turner opens that same door to jump to his death, the door is now firmly held in place by hinges. I say it may not be a mistake because some doors lift off their hinges but I can't see why the jump master would do that mid-flight as there is a hell of a slipstream and the door could easily be lost. (00:08:08 - 02:26:49)
Factual error: A number of the German uniforms in this film are incorrect for this period and location. The reversible white/camouflage parkas worn by the allies were scarcely seen beyond the eastern front before D-Day and would have looked very suspicious without the matching over-pants. Additionally, the marsh pattern camouflage on these didn't appear on these until mid 1944, which only partially replaced the Heer splinter patterns, SS camo patterns, as well as blue and olive solids. Finally, the stereotypical black SS uniforms seen in the film were largely replaced with an olive green version of similar cut nearly 10 years before the film was set. In 1943-44, it was only worn by Himmler (who also wore the olive version), his staff, very highly placed members of SS related organizations, some of Hitler's bodyguard detachment, and native security forces on the eastern front (hiwi's).
Continuity mistake: The camera is looking down on Smith as he climbs up the rope and there are no footprints in the snow on the roof of the cable car room below him. The camera then cuts away to different angles but when we see the same shot of Smith a few seconds later there are clear heavy foot prints in the snow on the roof. (01:09:40 - 01:10:10)
Continuity mistake: When Eastwood gets off the German motorbike after the escape from the town, the seat of his trousers are wet but 2 seconds later are bone dry.
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.