Other mistake: After the Cid dies and is propped up on his horse to try to fool the enemy into thinking that he still lives, he leads his army out of the fortress and attacks the offenders. Just after he comes out of the gates the horse stops and there is a shot of the Cid on his horse being almost silhouetted against the sun. If you watch carefully you will see Charlton Hestons' head move slightly and look towards the direction of the enemy.Even though the horse turns slightly under him the Cid's' head keeps looking in their direction. Obviously this would be impossible if he was dead, and even more so when you remember that his body was supposed to have been tied tightly to a support so that he would sit upright when on top of the horse.
Other mistake: The giant medallions that Pilate takes from a chest and then places on the pillar of the Temple do not bear close inspection as images of Tiberius, for some reason the name Augustus is lettered along with abbreviations not easily discerned as what was then used. The image behind Pilate when he is in court is similarly inauthentic but I would think that no-one but an exacting numismatist would even notice this if it is indeed such a major mistake.