Trivia: In the beginning of the movie, when the candy store owner is singing the candy man song, there's a spot towards the end of the song where he lifts up his counter to let all the kids in. Watch the little blonde girl as she gets walloped in the chin by the counter. (00:04:30)
Revealing mistake: When Violet turns into the blueberry you can see the reflection of the blue light in her hair. But in the same scenes where blue make-up is used instead of the light, you can see the regular flesh-tone under her hair and in her parting.
Revealing mistake: You can clearly see the string holding the elevator as it flies over the city. (01:33:00)
Revealing mistake: In the scene where Willy Wonka and the group of golden ticket holders enter the room with the chocolate river, Willy Wonka picks up a mushroom head with his cane. If you look closely, you can see holes under the mushroom head where he has previously done this, probably more than one take for that scene.
Revealing mistake: When Violet turns blue, in any shot facing her back you can see the blue light source used at the very left of the frame (widescreen only). (01:12:24)
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