Other mistake: When the boat that Willy Wonka and the passengers are riding on is heading for the tunnel through the river of chocolate, two crewmen are cranking the paddle wheel. The wheel is in an opening in the middle of the deck. As the paddle is rotating, moving the boat through the river, the blades are always clean and dry and never have any chocolate on them whatsoever.
Other mistake: In the scene where the newscaster is about to announce that the first gold ticket has been found, he walks over to a map of the world. Take a look at the northwest corner of the US. There is a big blue dot just to the right of Idaho that cuts through Montana and covers half of Wyoming. The dot is where the newscaster will later hang the number three marker.