Continuity mistake: During the dinner party scene, the positions of the shrimp which hang around the small colourful sushi bowls often change. Sometimes they hang over the bowls, sometimes inside the bowl. This happens with Delia's shrimps especially. (00:48:55)
Continuity mistake: During the scene where the Deitz family and their guests get possessed to sing and dance along to "Day-O," the positions of the black and white napkins, the wine glasses, the cutlery, the wine bottles in the ice-buckets, etc., change many times throughout this completely hilarious scene. (00:50:20)
Continuity mistake: During dinner, Otho is speaking and Delia listens, her hand away from the glass on the table. A shot later she is drinking from it. (00:51:20)
Continuity mistake: When Barbara picks Beetlejuice up from the model town, in one shot his arm is wildly shaking around in fury, but in the next close-up his arm is now tucked in between Barbara's fingers. (00:59:05)
Continuity mistake: When Charles and Delia are sitting out on the white terrace, Charles holds a glass with brown coloured drink in it which is three-quaters of the way full, and Delia's glass is half full. When Otho walks onto the terrace, Charles's drink is clear-coloured and the glass is filled right to the top, and Delia's glass is now three-quaters of the way full. (01:01:35)
Continuity mistake: When Otho is calling the spirits of Barbara and Adam to return we see Barbara first, suspended above the table in her wedding gown. She "ages" so rapidly during Otho's chant that her skin shrivels and her feet curl up, causing one of her shoes to fall off onto the table. A few minutes later we see a long shot of both Barbara and Adam suspended above the table, but her shoes are both on and there is nothing on the table. (01:13:30)
Continuity mistake: As Adam speeds towards Beetlejuice's foot in the miniature red car, the amount and position of plaster and rubble around Beetlejuice's foot changes between shots. (01:20:40)
Continuity mistake: In the wedding scene at the end when Barbara comes crashing through the ceiling on the sandworm, Beetlejuice looks up at the ceiling, which is still intact, and screams - a split second later Barbara actually crashes through and the sandworm devours Beetlejuice. (01:20:45)
Continuity mistake: When Lydia dances at the end, the two times we see the top of her head, her hair is parted differently. (01:23:20)
Continuity mistake: When Beetlejuice is in the waiting room at the end of the movie, he speaks to one man with a shrunken head, who does not reply. He then turns to the voodoo man sitting to his right, and he notices that this man has the a lower ticket number, so he steals this mans ticket and then throws his own ticket into the voodoo mans lap. But in the next shot the ticket is not in the voodoo mans lap, but it is on the seat beside his left leg. (01:24:15)
Continuity mistake: When the fly lands in the cemetery, the ground is some sort of spiked rubber. When the camera is flying in, we see the cemetery is actually dirt and sand glued to the board.
Continuity mistake: At the end when Beetlejuice is sitting next to the witch doctor he throws the number on his lap. When the doctor picks it up it's beside him on the chair.
Continuity mistake: After Betelgeuse knocks down the model tree, Adam places it back into its hole. Later on, as the camera zooms in on the whole model, you see the tree is still on the ground.
Continuity mistake: The photos Charles is holding of the Maitlands in sheets are 35mm photos based on the shape of the photo. The pictures Lydia took of them were Polaroid which are square and not as detailed or sharp as the other pictures, much less the same shape even if they were blown up.
Continuity mistake: When the wedding clothes are laid out for the seance, there are no pants/trousers in place for Adam, but when he materialises he has them on.
Continuity mistake: In the Day O scene. Lydia is sitting at the table next to her father. When the possession takes place, Lydia is not longer at the table and in a brief moment is seen in the corner hiding, laughing behind her chair. The camera closes in on the song and dance, and when pulls it back Lydia and her chair are gone and the corner is empty.
Continuity mistake: Adam picks up his head from the table and puts it back on. In the following shots the blood around the neck and head has disappeared.
Continuity mistake: At the start, when Adam drives to the store a fireman is cleaning the right side of the fire truck. A shot later he's now cleaning the front part. Also, when he exits the store the two firemen swap places between shots.
Continuity mistake: As Barbara recoils after getting hit with the steel plate over her mouth, she starts to move back just before, and repeats the move just after, it appears.
Continuity mistake: When Beetlejuice is screaming at Barbara and Adam, he kicks over the model tree. As he does, his hands are close to his sides. In the closer shot, they're a bit further out from his sides in preparation of the crotch grabbing joke.
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.