When the bad guy tries to attack King Arthur he is repelled by the sword. He runs out of the chamber and slams the heavy wooden doors closed behind him. As he does so three knights throw a spear at him. However on the other side of the doors, seven spears are shown piercing through it.
When Kayley and Garret are standing by the acid lake, deciding whether or not it's safe, Garret dips just the end of his walking stick into the lake, but when he pulls it out, almost half is gone and the end of what remains is charred black. In the next shot, the entire walking stick is complete and unharmed.
When Kayley and Garrett are crossing the acid pond on the bobbing rocks in Dragon Country, they show a top view of Garrett crossing and that he only has one rock until he reaches the edge. But then they show you the front view of Garrett crossing and it shows he has several more rocks to go before the edge.
In the second-to-the-last shot where King Arthur comes out of the keep after Excalibur is back in the stone, Garret's stick, which had previously been in two pieces in Arthur's projected path, scoots to the right and one piece (the top half) is missing.
When Lionel and the other knights go to Camelot, it takes a day for them to get there. When Arthur and the knights go to his house with his body, it might be implied that it took longer for them go there, as it is already winter.
When Julianna turns to the family painting on the wall, Kaylye's father is depicted as he was right before his death, as is Julianna. Kaylye on the other hand looks like a teenager instead of the small child she would have been when the painting would have been made.