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.
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.
Visible crew/equipment: When everybody first comes in the main room with the chocolate river, there is a shot of all of them standing in the doorway. At the bottom of the scene is the shadow of the camera.
Visible crew/equipment: In the Fizzy Lifting Drink scene, as Wonka leaves the room, a bubble machine is just visible at the bottom left of the screen.
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