In the long shot where Hank makes the sun "vanish" during an eclipse (a process shot), the sun is depicted as being covered by a black disk while the sky around it remains sky-blue; in reality, the sky would go dark, as if night had fallen.
In the joust between Hank and Lancelot, cranes are used to lift them to their horses. Those cranes - whose obvious purpose is to make Martin and Lancelot look utterly ridiculous - are copied from an scene in Olivier's Henry V, and are totally fictional; in reality, a full suit of armor did not weigh more than the full equipment of a modern day infantryman, and knights were drilled to be fully able to mount a horse without needing any silly mechanical aids.
Merlin is seen using a telescope toward the end of the film. The first person to apply for a patent for a telescope was a Dutch eyeglass maker named Hans Lippershey, in 1608. The use of lenses does do go back further than that. But, certainly would not have been seen in the sixth century, nearly a thousand years before.
When Hank is tied to the post for burning the guard holds the proclamation in front of him as he reads. His shadow is clearly visible on the paper, yet Hank, who is behind the paper and facing the sun, is able to focus the sun's rays on the paper with his magnifying glass, setting it on fire.
When Hank, Lady Alisande, King Arthur and Sir Sagramore are in the slave pen, Sir Sagramore grabs a guard and pulls him back against the bars. One of the heavy bars visibly flexes and then springs back, showing that it is made of rubber.