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.
Revealing mistake: Just before Charlie opens his winning Wonka bar, we see printed on the back label: "Packed by The Quaker Oats Company Chicago, Illinois". It is made clear in the film that no other companies are involved in the creation of Wonka's products - they're dispatched from his factory wrapped and ready to sell.
Revealing mistake: In the Chocolate Room, there are several see-through pipes coming out of the chocolate. The chocolate is supposed to be being sucked upwards in all of them. However, in some of the pipes it is actually flowing downwards, since it was easier for the film-makers to only pump it upwards in some of them and then send it into the other pipes to go back down. This is confirmed when Augustus Gloop gets stuck in one of the up-pipes - there is no chocolate at all flowing in one of the down-pipes while he is stuck in there.