Revealing mistake: When Violet turns into the blueberry you can see the reflection of the blue light in her hair. But in the same scenes where blue make-up is used instead of the light, you can see the regular flesh-tone under her hair and in her parting.
Revealing mistake: After Violet has chewed the gum and is blowing up, if you look near the bottom of her right leg, you can see the air hose that is blowing her up. Visible on full screen only.
Revealing mistake: You can clearly see the string holding the elevator as it flies over the city. (01:33:00)
Revealing mistake: In the scene where Charlie and his grandfather are floating from the fizzy lifting drinks, you can see the black harness Charlie is wearing.
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: When Wonka transports the giant chocolate bar, the stand the chocolate lies on inside the TV is visible.
Revealing mistake: At the beginning, in the candy factory, Bill (the store owner) is handing out candy left and right and singing to the kids. There is a shot where he turns around and a girl is pretending to throw candy into her mouth. Clearly her hand is empty.
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. (00:34:00)
Revealing mistake: Right before Augustus goes up the pipe, there is a long shot of the pipe. Look very closely. You will see that there is a dummy of Augustus, not the real Augustus.
Revealing mistake: When the Candy Man is singing his song, he tears a long paper strip with candy dots on it. Look closely, some of the children aren't even taking them off to eat them; they're only pretending to eat them.
Revealing mistake: When Violet turns blue, in any shot facing her back you can see the blue light source used at the very left of the frame (widescreen only). (01:12:24)
Revealing mistake: When Mrs. Teevee puts the shrunken Mike into her purse, it is a very obvious fake Mike.