Continuity mistake: When Otho is going through the house suggesting changes he is wearing white sneakers in one room and red ones in the next. (00:16:25)
Visible crew/equipment: When Adam, Barbara and Juno ascend a staircase, talking about how Adam and Barbara should start more simply if they want to scare the Deitz family out of their home, there are two tape markers - a yellow and a blue one, very clearly on one of the steps. (00:34:35)
Continuity mistake: When Adam creates the door to gain access to the spiritual world, he draws the outline and door knob quite visibly with chalk. Yet when the door is closing after they have gone through it, though you can still see the outline of the door, the door knob has mysteriously vanished. (00:26:15)
Visible crew/equipment: When Barbara tries to leave the house and falls into the sandworm's world you can clearly see her knee pads. (00:19:00)
Visible crew/equipment: When Otho is doing the Seance, Adam and Barbara appear in their wedding clothes and then float above the table. When Lydia releases Beetlejuice, everyone gets up and leaves the table, and in this wideshot the black wires that Adam and Barbara are suspended from are visible. (01:15:25)
Continuity mistake: After Adam has drawn the chalk door on the wall, he walks forward and closes the “Handbook for the Recently Deceased.” In the next shot, as he knocks on the chalk door three times, the handbook is now open. In the next shot as he steps back, the handbook is now closed again. (00:26:20)
Continuity mistake: During the dinner party, in one shot from behind we see Otho has very little wine in his glass, but in the following shot of him from in front, his glass is now nearly full. Nobody refilled it between the shots. (00:48:55)
Continuity mistake: When Delia spray paints the word “mauve” on the wall, between shots it is apparent that the script of the word changes. One example would be that in the first shot the tops of the letter “M” are pointed, like triangle tips, but in the next they are more rounder. The writing also moves lower on the wall. (00:15:50)
Revealing mistake: At the end, as Beetlejuice's head shrinks, the collar has a line around it, showing where the two images were superimposed.
Revealing mistake: At the start, after the car veers away from hitting the dog and comes to rest teetering on the edge of the bridge, you can see the quick release wire cable that was used to secure the car to the bridge. It's also very visible when the car is sinking into the water. (00:07:50)
Revealing mistake: When Maxie and Juno are jetted through the ceiling, check the stiffness of their bodies: they've been replaced by mannequins. (01:19:20)
Answer: They died by drowning. Jane's daughter even tells Lydia when she asks what happened. Although Adam and Barbara should have been completely wet for the whole movie, Tim Burton decided to keep Alec and Geena dry as he felt that keeping them wet the entire time would be an uncomfortable experience for both of them.