Continuity mistake: In the contract signing scene, Violet and her father are close to Wonka's right elbow, but when the shot changes to Veruca and her father's view, they are further away, then close to Wonka again in a later shot.
Revealing mistake: When Charlie and Grandpa Joe drink the fizzy lifting drink and are floating in the air you can see the hook that is connected to the wire that Charlie is being held up by.
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.
Continuity mistake: At the Everlasting Gobstopper machine, Veruca Salt arrives before her father, and when Wonka announces the name of the candy, she and her father turn to each other and exchange glances. However, the next shot shows Veruca's dad just arriving at the machine and joining her.
Revealing mistake: You can clearly see the string holding the elevator as it flies over the city. (01:33:00)
Continuity mistake: When Mr. Wonka is first walking toward the crowd, his walk is much longer than the actual path. Watch the shadows change on the overhead shots as the day of filming went by.
Continuity mistake: When they're in the room with Wonkavision, as Mike Teevee jumps up on the platform to get shrunk, in the background an Oompa Loompa puts goggles on, then in the very next shot he puts them on again, with no time to have taken them back off.
Revealing mistake: In the Egg Room, when the golden eggs are falling down onto the scale, and particularly when the egg Oompa lifts one onto the cart, you can see the lining on their sides, even though they are supposed to be actual goose eggs.
Continuity mistake: When Willy Wonka presses the button for the Spectacular Gum Making Machine, all of its gears and parts and contraptions start working. In one shot, a tray of a pale white chunky substance is covered by a lid. As the lid lifts off of the tray, the pale white chunks are now bright colors. You can see where the cut in the shot was made for them to replace the first tray with the second tray. There is a slight jump in the position of the lid and the misty fog in the shot changes in the same instant.
Continuity mistake: If you look closely at several scenes, Veruca Salt's and Violet Beauregard's hairstyles change. For example, Violet's hair is wavy, then it is even curlier.
Continuity mistake: When Grandpa Joe gives Charlie the candy bar bought with the tobacco money, Joe pulls it out, holding it at the top right and bottom left corners. When the shot cuts to Charlie and he says "Grandpa, that money was for tobacco." you see Grandpa Joe in the foreground of the shot, and his right hand has moved from the top right corner to the lower half of the candy bar. When the shot cuts back, his hands are as they were before, on the top right and bottom left corners of the candy bar.
Audio problem: In the "Candyman" song near the beginning of the movie, the candy store owner is on a ladder, about to drop candy into the kids' hands. When we see the kids holding up their hands and saying, "Me. Me.", none of their mouths are actually saying anything. (00:04:00)
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.
Audio problem: During Veruca's song her lips don't match what she's saying.
Revealing mistake: When Wonka transports the giant chocolate bar, the stand the chocolate lies on inside the TV is visible.
Audio problem: When Charlie and Grandpa Joe are first being lifted into the air after drinking the fizzy-lifting drinks; Charlie tells his Grandpa that it's ok. But instead of saying "Grandpa", he very clearly says "Grandma".
Continuity mistake: When Wonka's boat is about to depart, two shots are cut together, but the sound recorded of the second is used over both. This results in a clear view of the Oompa Loompa steering the boat with the boat's bell (and the rope to ring it) not being rung by anyone whilst you can hear it ringing - then the shot suddenly changes, and the Oompa Loompa is obviously ringing it in time to the sound.
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)
Answer: Closed circuit security was invented in 1942 and came into common use in the late 60s and early 70s. Beyond that, Wonka could have had Oompa Loompas monitoring the group or simply noticed that they were gone and guessed.
Greg Dwyer