Revealing mistake: In the scene were Charlton Heston has just said goodbye to Sophia Loren and is about to ride off with his followers (last scene just before the set intermission in the film), you can just make out a tiny little red automobile riding through the landscape in the background.
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.