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: When Caesar's ship arrives in the Alexandria marketplace, he tells his men to "go marketing" rather than force their way through the crowd with swords. Rufio says "you're not serious?" He then silently mouths along with Caesar's reply, "We shall shop our way to the palace steps." (00:11:30)