Continuity mistake: When Dennis first meets the man who cut his own foot off, it's because he shoved his hand (with a cup in it) right in front of his face. When the shot changes so we can look down at who it was, we see that his hand is nowhere near Dennis' face (he would have had to have been standing up and leaning over to reach his face, when we see him he's sitting down and leaning back against a wall).
Continuity mistake: The peasant with the rock is killed by the Jabberwock. When his skeleton first lands next the Dennis, it has a bit of flesh left on it but lots of bone is visible. The shot then changes to show Dennis backing away and the peasant's corpse now has a lot more flesh on it and the legs are in a different position. (00:31:15)
Factual error: In his fight against Dennis and the Jabberwocky, the Black Knight uses a flamberge, a very distinct two-handed sword (note the little "crescent" hooks on the blade). The only problem is that these swords are a German design from the 15th century, more than 200 years after the movie is said to take place.
Factual error: The movie takes place in a Medieval European setting. This general historical context is strictly observed, but with one exception: the potato. Potatoes are part of the so-called Columbian exchange. There were no potatoes in the Middle Ages.
Visible crew/equipment: When the Black Knight falls off his horse (the same scene where you can see a truck in the background), to the right of the screen you can see a man who is wearing a modern day brown suit and a cap, standing with his hands in his pockets and watching the action. (01:29:25)
Continuity mistake: Just before Dennis rescues the Fishfinger family, the knight is riding the horse that the Dog knight was riding earlier. When the Knight and Dennis are walking through the destroyed village the knight is riding a different horse. The pattern of white on its nose is very different. (01:23:15 - 01:29:35)