Deliberate mistake: When Sunny picks up the Scrabble letter, you can hear her biting a piece of it. But when she places it on the board, you can see it hasn't been chewed on. It looks exactly the same as when she picked it up. (00:03:45)
Deliberate mistake: In the shot where the camera zooms in on the car, after they realise they are standing on a train track, it is obvious that the car would be hit by the train even if they managed to change tracks. The car's hood is parked right above the other track. (00:23:45)
Deliberate mistake: When Violet and Klaus have the anchor on the three cans they use to roll it on, just before they tip it over the edge, it is obvious the anchor isn't a real anchor. It's too light. When they keep pushing it back and forth, it should tip over because of the weight if it was a real anchor. They shouldn't have been able to handle it like that. (01:01:45)
Chosen answer: The spy glasses are not explained in the books, but it seems that these are symbols of the fact that they are in VFD, the secret organization we learn about later on in the series. Both Dr. Montgomery and Aunt Josephine's husband and brother-in-law, the Anwhistle brothers, are implied to have been a part of VFD.