Revealing mistake: During the chariot race, where the contestants are riding towards the screen, tyre tracks from the camera vehicle are clearly visible imprinted in the sand. (00:20:50)
Revealing mistake: When one of the ships collides with the other, you can clearly see that the rowers being hit are dummies. (01:19:00)
Revealing mistake: There is a hex bolt holding the spikes to the chariot wheel hub. (00:14:10)
Continuity mistake: When Judah takes rest on his way to Jerusalem funny things happen with his headgear. First, when he lies down it suddenly disappears under his head. Then he falls asleep, and when Balthasar from Alexandria approaches him a little later and he quickly gets up there is nothing under his head and back. (01:38:00)
Continuity mistake: Count the number of chariots that start the race. Then count the chariots that get thrown out and the number that finish - there's one too many. There are 9 at the start, 6 get thrown out, and 4 finish.
Continuity mistake: One of the first shots in the picture is a wide angle establishing shot showing a street scene in Jerusalem, with milling people, animals, etc. A tall figure in the background is slowly walking toward the camera. The figure is Charlton Heston, and the shot is an earlier segment of a shot used later in the movie when Ben-Hur is returning home from a trade expedition. Heston's costume is the same, but he's only clearly recognizable in the second sequence. Since the two scenes are set 33 years apart, perhaps the earlier figure is Ben-Hur's father. (00:07:40)
Deliberate mistake: During the chariot race, the chariots crash together but the arrangement of the horses make it impossible for them to actually collide. In some instances (particularly in an aerial shot as Ben Hur and Messala round the turn) it is obvious that they repositioned the shaft of the chariot to between the first and third horses instead of being in the centre between the second and third.
Continuity mistake: During Ben Hur and Messala's duel on the chariots, Messala's wheel-scythes tear away most of the right hand side of Ben Hur's chariot. When Messala has been thrown and Ben Hur crosses the finish line a few moments later, however, the camera has zoomed out far enough to reveal that his chariot is perfectly intact.
Factual error: The actors and stunt drivers use only two reins to control the four horses that pull each chariot in the race. However, charioteers in ancient Rome used separate harnesses with two reins controlling each horse. In a race such as the one depicted in the movie, where four horses draw the chariot (two-horse races were common, as well), a real charioteer from the era would be holding eight reins.
Factual error: The galley during the battle scene is rowed by slaves. Romans during this time period didn't use slaves to row their galleys. Rowers had to be able fight. If not, the Romans would be outnumbered 200 to 30 when they boarded the pirate ship as the pirates rowers would be able to fight.
Audio problem: When the rowers in the Roman galley were going normal speed, you could tell that the drum was sounding before the drummer hit it.
Continuity mistake: At the end of the chariot race, a heavily bloodsoaked Messala is taken to receive medical treatment. While something is done to his legs out of shot (amputation, presumably) an orderly in a clean white tunic leans his whole body weight on Messala to hold him down on the bed as he writhes in agony. When the orderly stands up, we see that his tunic is still spotlessly white.