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 James Coburn and Charles Bronson are pretending to be Russian prisoners and attempting to walk out of the camp, if you look in the open windows of the huts in the background (past the men leaning out the windows), in several of the huts you can see that there is nothing inside, with wooden beams/braces visible to hold up the walls.
Revealing mistake: When Hilts is first locked in the cooler he turns to verify the door is locked. Apparently the set wall was so flimsy the stage hand seen on the left was required to hold the wall as Hilts pushed the door, to prevent it from moving. The stage hand looks like the German guard without his rifle.
Revealing mistake: When McQueen is jumping the fence on the motorcycle, it can clearly be seen the machine used for the jump is in fact a modern offroad model, with long travel suspension, in camouflage guise.
Revealing mistake: In the scene where the goons find 'Tom', the water that is poured down the hole is not steaming, even though the water burnt his hand.