Continuity mistake: When Charlie's grandad falls on him soon after getting news of his Golden Ticket win, one of the other old guys in the bed has a shocked look on his face. In the next shot however, the old guy now has his hand over his face and not enough time has passed for this to occur.
Continuity mistake: When the kids stick out their hands to receive gobstoppers, Mr. Salt, who is standing behind them, changes position between shots.
Continuity mistake: When Willy Wonka comes out of the factory to greet the children who will be touring that day, he walks down a red carpet. A minute or so later the lucky children walk up the same carpet. As the scenes cut back and forth between Wonka and the children the viewer sees a shadow on the carpet of the building next door. At one point the shadow has shifted noticeably and then reverts to its former position.
Continuity mistake: When Charlie first comes home after he's got the golden ticket, he hands the ticket to his grandfather to read. After he says, "You've done it Charlie." and hands the ticket back to Charlie, look carefully. The ticket is thinner, less wrinkled, and doesn't have any writing on it.
Continuity mistake: When Charlie visits his mother at the Laundry, he sits down at the washing bowl. When you see Charlie from behind you can see the big wooden spoon sticking out of the bowl. When the shot turns to the front of Charlie, he is stirring the laundry with the spoon. In the next shot of Charlie from behind the spoon is in the exact same spot as previously - it hasn't moved.
Continuity mistake: While Charlie and Grandpa Joe are floating toward the fan, it is spinning in a clockwise direction; seconds later, when they get dangerously close to the fan, it is now turning counter-clockwise.
Continuity mistake: At the end of Veruca's song she sings "Don't care how, I want it now". In one shot her hands are at the far sides of the box, but in the next shot they are together at her sides.
Other mistake: In the scene where the newscaster is about to announce that the first gold ticket has been found, he walks over to a map of the world. Take a look at the northwest corner of the US. There is a big blue dot just to the right of Idaho that cuts through Montana and covers half of Wyoming. The dot is where the newscaster will later hang the number three marker.
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.
Audio problem: When Mr. Wonka bites into a teacup at the end of the song of the room with the Chocolate River, you hear a crunching sound, yet it looks like he has to "tear" the cup a bit. How can he crunch on somethng that requires him to tear at it? [In an interview, Gene Wilder stated the cup was made out of wax - not excusing the mistake, but in case anyone was wondering...]
Continuity mistake: When Charlie is working with Mr. Turpentine on the wart-removing potion, the bottles and cups on the table change between shots.
Revealing mistake: When Mrs. Teevee puts the shrunken Mike into her purse, it is a very obvious fake Mike.
Continuity mistake: When the first "good egg" falls on the eggdacator, its position changes between shots.
Audio problem: When the red shirt kid comes into the classroom and talks about the tickets, in the closeup, he's facing inward. When he says "You've got to buy Wonka bars", and the shot goes wide, he's facing the door, turning to his right to leave. His mouth doesn't appear to be moving.
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.
Visible crew/equipment: In the invention room, Veruca's dad talks to Wonka. The camera gets close to them. On the bottom of the frame is either a shadow of the camera or the boom mic.
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.
Revealing mistake: As Veruca sings "I want it now" in the egg room, the plastic seams on a few of the golden eggs are quite visible, along with their 'dimpling' near the tops and bottoms, as the Oompa Loopmas pass them around for polishing.
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