Revealing mistake: During the dancing scene, Jamal shows the percussionist how to use the tambourine first, then tells him to start drumming, so he puts the tambourine down. However when he cues in the guitar, the tambourine starts to play again with the drums and guitar. A few shots later, with the tambourine still playing in the background, you can see the percussionist's tambourine sitting on the table next to him where he had left it.
Continuity mistake: There is a scene about 2/3 of the way in the movie where you see Jamal (Martin) standing by the lake and his chain is under his shirt, he looks back into the forest and his chain is hanging out of his shirt, then he goes back to the lake and his chain is under his shirt again.
Other mistake: When Martin Lawrence falls over the railing in attempt to get the mysterious necklace, the camera goes to another shot that pulls away from the moat. You can see the logo on the back of Lawerence's cover-alls swimming away at the corner of the screen, when he's supposed to be under water.
Continuity mistake: When Jamal is trying to ride a horse for the first time, he rides to a stable. When Jamal leaves the stable, he has wheat on his shoulders. He takes it off, but in the next shot he has wheat on his shoulders again. Then in the next shot he does not have wheat on his shoulders anymore.
Factual error: Towards the end, when the medics are reviving Martin Lawrence, they are using a defibrillator. You can hear the sounds of the defibrillator charging and then discharging. They then show Martin Lawrence wake up, but he is fully dressed. In order for a defibrillator to work, it must come in contact with the skin. I doubt they had enough time to redress him after using it on him.
Plot hole: The legend of the black knight (in this film) is that he can breathe dragon's fire. Jamal somehow rigs up his aerosol spray-can inside his helmet to shoot out the flames. However, there are three problems with this: one, how did it fit in the helmet? Two, how did he activate the spraying mechanism without his hands? And three, how did he even ignite the spray?