Revealing mistake: In the scene where Willy Wonka and the group of golden ticket holders enter the room with the chocolate river, Willy Wonka picks up a mushroom head with his cane. If you look closely, you can see holes under the mushroom head where he has previously done this, probably more than one take for that scene.
Continuity mistake: On the closeup with Sid sitting on the chair, you see two knitting needles, one on either side of his jacket. When Hatty and Sid are in view, the jacket completely covers the knitting needles, next shot, closeup again, needles seen again. (00:18:40)
Visible crew/equipment: When the Mayflower truck crashes into the police cars, crew members and the second camera (presumably filming the stunt from the overhead angle) can be seen in the wideshot at the left side of the screen, before the camera pans to the right for a close-up of squad car 351.
Visible crew/equipment: When Mr. Mainwaring, Mr. Wilson and Pike have left the radio shop and they are walking up the street, you can see the crew in the reflection of the shop windows.