Continuity mistake: During the enactment of the Battle of Zama, Maximus rides his horse around a bend with his sword in his left hand. When we cut to close up, it's back in his right. Looking at the left-handed shot in freeze-frame, you can see the frame has been flipped, left-to-right (check the armor). This was probably for continuity reasons. Similarly, Maximus jumps on the horse and his shoulder protector is clearly on his injured left shoulder. However, a few frames later, the protector is on his right shoulder, then it moves yet again back the correct spot, the left shoulder. (01:24:45)
Continuity mistake: Gabriel writes in his letter that his friend was killed at Elizabethtown, but you can see that the letter actually states he was killed at Monmouth.
Continuity mistake: The soldiers takes off from the aircraft carrier in Boeing CH-46 helicopters. In the next scene they are flying towards the target and the helicopters are magically upgraded to the Boeing CH-47 model. The mistake is clearly seen when counting the landing gear on the two types. The CH-46 has a single front landing gear and the CH-47 has a double landing gear in the front.
Continuity mistake: In the scene where Miter is planning to escape from the army and Bozz tries to stop him, watch the red mark on Miter's neck. It miraculously disappears and reappears many times. (00:45:05 - 00:48:30)