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.
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.
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.
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.
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.
Continuity mistake: On the boat ride, when Wonka explains about "The Sailor's Way", his head changes position between shots.
Continuity mistake: After introducing Mr. Wilkerson, Wonka's hands magically appear on Charlie's shoulders.
Continuity mistake: When Veruca tells her dad that she wants a golden goose the stuff in the bowls behind them keeps changing.
Continuity mistake: When Wonka is leading the group down the stairs into his factory, Veruca's right arm changes position from her hand on the bannister to her entire forearm resting on it.
Continuity mistake: A tree suddenly appears behind Grandpa Joe, Charlie, and the Beauregard's that wasn't there before, as they rise to see Wonka emerge from his factory.
Continuity mistake: After Wonka tells Mr. Salt a waterfall mixing chocolate gets it just right, both men are very close together, but the next shot of them shows Salt further away from Wonka than before.
Continuity mistake: In the scene after Charlie brings home the loaf of bread, his mom moves in front of the television set near Grandparents George and Georgina, but the closeup of George and Georgina shows Charlie's mom nowhere near the TV set.
Continuity mistake: When Wonka enters the shrinking corridor, he pauses in front of the wooden door, to the right of the viewer but the next shot shows him further to the left.
Continuity mistake: When Charlie gives his pay to his mom, Grandparents George and Georgina are asleep but their head still change from lying at an angle to straight as the shots change.
Continuity mistake: Outside the Inventing Room, Veruca and her father stand about a foot apart from each other, but the next quick shot shows Veruca standing in front of her dad's left arm.
Continuity mistake: When Augustus falls into the chocolate river, Wonka's right hand is resting on the top of his cane, which is planted in the ground, but the next shot shows his right arm up, and bent at the elbow.
Continuity mistake: When Veruca throws the golden wrap at Wonka, the table holding the wrapper is head-on to the viewer, implying Wonka's point of view, but after Veruca throws the wrapper on Wonka, the table is off screen and 90° to the left of Wonka and the other factory guests.
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