Revealing mistake: When McKay and Ramon are riding into Blanco Canyon to rescue Julie, you can see a diesel loading shovel with its bucket raised in the bottom of the frame of one of the scenic shots. Presumably the scene was shot in a quarry and they didn't get all the equipment out.
Revealing mistake: Joe is much too old to be in high school. He appears to be in his thirties (the actor playing him was 35 at the time).
Revealing mistake: When Morgana's ship is being boarded, one of the Vikings features an arrow stuck in his body even as he boards. It is first (but not quite) kept out of sight by his shield, but when he enters the ship, he turns the shield away, and the arrow is suddenly visible. Somewhat hard to see by casual watching, so look carefully.
Revealing mistake: In the final scene in the belltower you can see shadows from the surrounding trees. That's because this scene was shot at 3 in the afternoon. They just used a filter to make it look like it was night.
Revealing mistake: When Corporal Bin's group of men eventually arrive at Dunkirk, they speak to a soldier adding sandbags to a Vicker's gun position. Here (and later on in the film when it is shown firing) it has a blank-firing adaptor prominently fitted to the front. Additionally the belt of ammunition shown going through it is filled with blanks - no bullets project outside the other side to that of the actual cartridge cases.
Revealing mistake: When Van Helsing arrives at Castle Dracula and discovers the Count's bride lying staked in her coffin, the "corpse" is visibly breathing. (00:29:10)
Revealing mistake: Whenever our heroes are shown firing the Vickers guns on or off the vehicles, they are very obviously fitted with blank-fire adapters to the front. The belts of ammunition for them throughout the film are also blanks - the cases are visible, but the bullets that should project out the other side of the belt are not present.
Revealing mistake: When a yellow cab pulls up to the curb, it is obvious that the street is part of a studio backlot and not a real city street because the businesses have no names. The sign outside one building says simply "Tobacco." The diner has only the word "Restaurant" painted on its windows. Another business has no identification other than "Fish and Poultry" painted on its window.
Revealing mistake: When the traveller crosses the drawbridge towards the end of the cartoon, the edge of the cel is visible.
Revealing mistake: In the opening credits, Richard Widmark is driving a car with Doris Day in the passenger seat. Widmark does not turn the steering wheel, even though the rear-screen projection shows him going around a curve.