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: At the lagoon, after Price and Brown win the cup and the hook falls, Paul is suddenly replaced with a completely different kid, older and much taller.