Continuity mistake: When JB dreams about singing "Master Exploder" in the small bar, he points at one guy in the audience and literally blows his mind. When he starts pointing you can not only see JB's face, but also how his elbow is in a slight angle. In the next shot the camera shows the guy JB is pointing at and his elbow is completely stretched. The views change several times and the angle of his elbow changes after every cut, until the guy's brain is finally blown out.
Plot hole: There are six metal bars mounted around the circumference of the water wheel's axle. In the wheel's interior, when Jack frees himself he lands on his feet, begins to run and promptly hits his head on one of those axle bars, then falls out. Moments later, when Jack returns and runs in the wheel, he is shorter (way more than a foot) than all six spinning axle bars, which would make that previous shot impossible, however humorous it is. (01:52:40 - 01:53:55)
Revealing mistake: When Troy says "I'll sing with her," Ms. Darbus turns around "in response" to his voice before he even says anything.
Continuity mistake: Right after Queen Latifah hits her head and is having a fantasy, LL Cool J is unbuttoning his shirt. The scene cuts to Queen Latifah and when it cuts back his shirt is buttoned up and his tie is tied properly. It cuts back to Queen Latifah and back to LL Cool J where his shirt is now once again completely unbuttoned. (00:14:00)
Continuity mistake: When everybody is at the rehearsal for Grant and Julia's wedding, when Bernie does a toast to Julia and Grant, watch the level of water in Roz's glass, it changes without her even touching it.
Plot hole: Christopher Columbus speaks Italian while the Huns supposedly speak their native language. Shouldn't Octavius and his Romans speak Latin? (00:34:50)
Other mistake: While the wizard is speaking near the portal, a ghost image of him appears to the left.
Continuity mistake: When Ashley moves in with her two friends she breaks their bathroom mirror with a hairdryer. Later on in the movie, in the scene where her two friends are giving her a pep talk about luck, we see the same bathroom mirror in the background, which is now perfectly fine. Right after that, Ashley goes into the bathroom and looks at herself in the mirror, which has suddenly gone back to being broken.
Continuity mistake: Mina Hoyt's bra strap before the vault was a white strap. After she completed the vault, the bra strap changed to hot pink. (01:19:05 - 01:20:00)
Factual error: In a few scenes at the hotel by Heathrow Airport, several British Airways aircraft are seen, all with the current paint scheme instead of 1995's.