Continuity mistake: As the case of wonka bars is being auctioned, and the auctioneer says "3000 pounds, 4500 pounds", his arms go from being at the edge of the table to next to his sides.
Continuity mistake: At the start of the movie, when the school children are running out of the school towards the candy store, the time visible on the school clock is 3 o'clock. At Bill's candy store right next door the time is 3.20.
Continuity mistake: After Charlie gets the chocolate bar for his birthday the position of his fingers on the bar change between shots.
Continuity mistake: When Charlie is working with Mr. Turpentine on the wart-removing potion, the bottles and cups on the table change between shots.
Continuity mistake: When the first "good egg" falls on the eggdacator, its position changes between shots.
Continuity mistake: When Charlie sees the newspaper about how the fifth ticket was faked, the position of the newspaper in the man's hands changes between shots.
Continuity mistake: When WIlly Wonka is talking to Mrs Teevee about Loompaland (at the "Mr Wonka I am a teacher of Geography", "Oh good then you'll know all about it" part), the rear shot of Mrs Teevee shows Mr Beauregarde proceed to eat a giant sweet of some sort. But in the next shot he has the sweet at waist height.
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