Revealing mistake: Just before the Roman soldiers pressed Simon of Cyrene to help Jesus carry the cross, Jesus has fallen and we see that the bruised and swollen-shut eye is now his left. The shot changes and when it comes back to Jesus his right eye is swollen shut again. The film must have been flipped.
Revealing mistake: During the scene where the Romans soldiers raise Jesus' cross, they use two ropes on each arm of it. If you look at the rope on the left side of the screen, you can see that it is slack, it's obviously not supporting the weight of the cross. There must have been some device lifting it off screen.
Revealing mistake: This concerns the criminal to Jesus' left on the cross. If you pay close attention to the criminal's right hand, you can tell that the part of his hand that is nailed (past the area of the thumb) is obviously not his. When he moves around this part doesn't, and his wrist moves independently from the rest of his hand. Difficult to explain, but it is there.