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.
Continuity mistake: In the "I Want it Now" musical number, Veruca grabs some golden egg wrapper and throws it on Wonka. When she does, the table with the wrapping is immediately behind her, but when the scene cuts to her after she has thrown the wrapper on Wonka, the table has disappeared.
Continuity mistake: When the guests enters Wonka's Fizzy Lifting room, Veruca Salt and Mike Teevee are standing next to each other, when Mike turns away from Veruca and takes a step. The next shot shows them both close together again.
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