Revealing mistake: Towards the end of the film, when Hilts (Steve McQueen) is in a motorcycle chase with the German guards, if you watch closely, when the camera cuts away from Hilts and to the German motorcyclists, you might be able to notice that one of the guards chasing McQueen IS actually McQueen! They used him to play one (or maybe more) of the guards because of the experience he had with motorcycles. This can also be seen in some behind the scenes footage on the DVD version of the film.
Revealing mistake: After Jason's fight with Acastus, when Medea is telling Jason of the flower which can heal his wound, if you look past Medea you can see a mountain in the distance. On top of the mountain you can see a lighted building of some sort that appears to be a modern structure. (01:15:05)Eric Siegel