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)
Audio problem: During Veruca's song her lips don't match what she's saying.
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".
Audio problem: When Charlie and Grandpa Joe are singing the "Golden Ticket" song, Grandpa Joe says, "Watch my speed!", but his mouth does not move to match the words.
Audio problem: When the group is on Wonka's Pop-mobile and covered in the foam, Mike Teevee's mom says her line, "It's even in my shoes." From this point to the end of the shot, her lips don't match what she says and neither do Mike's, but the mom is more noticeable.
Audio problem: When the crowd is mobbing Charlie after he finds the Golden Ticket, the man with grey hair says "Come on Charlie, hold onto it" but his lips don't move.
Audio problem: When Charlie and his grandpa are burping to get down after drinking the fizz drink, their mouths don't match up with the times or lengths of their burps. When Charlie gets his feet on the ground he takes a last burp and his mouth doesn't even move.
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...]
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.
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