Otho's shoes when he is spray-painting the walls in the house change from fire-engine red elf-looking shoes to white trainers as he walks through the bathroom and then they change back to the red elf shoes as he enters the next room.
Otho opens a can of spray-paint and hands it to Delia, who opens it again.
After Barbara pulls Adam back on the porch, she shows him that they are not reflected in the mirror, yet, in the background, Adam's shirt is partly reflected in the window.
When the sculpture crashes through the kitchen window, the interior shot does not match the exterior. (From the inside, the kitchen has a doorway to the outside of the house, as seen at the start of the movie when Jane comes to visit, but from the outside, the window is much higher up in the house, where no entrance could come in.)
When the Maitlands are in the model of the cemetery, there is fake green grass. When they return to full size and look down at the model, the cemetery is covered with sand, not fake grass.
The morning after Lydia has taken photos of the Maitlands wearing sheets, she shows 8x10 enlargements of them to her stepmother in the kitchen. The pictures Lydia took were self-developing Polaroids, and there would be no negatives to make enlargements from.
Earlier in the film, the Maitlands see that they have no reflections on the mirror and Barbara holds up a toy horse by the mirror, showing it floating in the air, but later when they try to scare Delia and Otho, Barbara is holding onto a knife making it look like she cut Adam's head off. Surely Delia, Otho and Charles could've seen the knife floating in the air.
When Lydia starts taking Polaroid pictures of the Maitlands, she takes 12 pictures in 10 seconds. First, the motorized version of the Polaroid camera, the SX-70, takes, processes, and spits out one picture in about 3 seconds, so it would take at least 30 seconds to take all the pictures. Second, a pack of film for the SX-70 only contains 10 pictures. Third, for each picture taken, the images would take at least 30 to 45 seconds to become visible. Fourth, the flash also has only 10 bulbs.
When Otho, Charles and the two movers (in blue coveralls) remove the model from the attic, the layout is much wider than the doorway. If they turned it sideways to get it out many of the buildings and other things, such as the pickup truck, would fall off.
When the Maitlands are digging through cardboard inside the model to find Beetlejuice, one of the pieces of cardboard has "FRAGILE" written on it. If the model is a miniature you would not be able to see the whole of the word FRAGILE on one tiny piece.
All the people in the waiting room and in the office are in the same condition as when they died and the way they died is shown clearly. However, the Maitlands, who were drowned, are not wet. This was an artistic decision so that the actors would not have to constantly wear wet clothing.
In the dinner scene, Lydia can be seen to push her chair back to leave; she doesn't just disappear when everyone starts singing. Lydia doesn't leave the room. She slips out of her chair and the pulls it away with her to the corner of the dining room where she enjoys the Calypso fun.
Even though Adam and Barbara attempt to scare the Deetzes by altering their appearances, only to find that the living cannot normally see the deceased, later, when they show Juno how they plan on scaring the family again, they go back to altering their bodies, which Juno finds no flaw in. However, she told them to study the "intermediate interface chapter on haunting" and to practice, which, given the shrimp scene, they obviously had been doing both.
When Adam and Barbara first get to their house after dying, we can see their reflection in a mirror in a hallway to the left of them. A couple shots after that we see that they are not supposed to have reflections.
As the Maitlands are climbing the steps of the remodeled house with Juno, the cast's tape "mark" is visible on the step.
When Beetlejuice rises from his grave and flies after the Maitlands, the wires supporting him are visible.
When Barbara tries to leave the house and falls face down in the sand (the sand worm scene), she is clearly wearing knee pads under her dress.
Looking into the mirror, the ghosts prove that they have no reflections, but are later reflected in the attic windows.
When Adam, Barbara, and Juno walk into the remodeled house's attic together, and Juno says, "you should have been studying those lessons since day one", her lips don't move.
When Beetlejuice disrupts the séance, the wires holding up Barbara and Adam are clearly visible over the green-lit backdrop.
Right after the Maitland's car falls into the river, a rope tied to the bumper can be seen.
When Betelgeuse first appears to land in front of the Maitlands, after escaping the graveyard, cables can been seen attached to his hips.
When Beetlejuice has his first actual conversation with Lydia, a cockroach scampers by and he eats it. The cockroach is roughly the size of a hamburger in comparison to Beetlejuice, yet he is already miniaturized.
Towards the end, when Barbara is trying to stop the wedding of Beetlejuice and Lydia by saying his name three times, she pauses after the first one, giving him time to act, when she should've just said it three times rapidly, like she did when he was the giant snake, so he wouldn't have time to do anything.
Early in the film, (after the decapitation scene) Barbara waves in front of Charles and she states, "What's the good of being a ghost if you can't frighten, people away". Charles can't hear her. But during the bed sheet's scene, it's obvious the living can hear Barbara. Barbara and Adam moan, Delia hears it and turns off the TV. Also, it's obvious the Maitlands can manipulate physical objects. Why don't they just throw stuff at the Deetz, to get them out!!!!.
It is never explained why The Handbook for the Recently Deceased would appear to the living.
The book for the recently deceased has the process for which the living can exorcise the dead.
When Delia gets trapped against the house by her art, the "extra" with the curly hair and glasses looks briefly directly at the camera before looking back down.
When Otho summons the Maitlands into their wedding clothes, they start to age rapidly. Their age isn't always consistent in the following scenes.
When Lydia is writing her note and pulls off the top sheet you can briefly see the same wording 'I am alone' in pen in a new section of the paper. Not to be confused with the pen indent on the page underneath. Likely a prepped prop for multiple takes.
When Lydia is talking to Beetlejuice in the model, he says, "Come to think of it, I don't have any rules." But later, at the "wedding," he warns the preacher, "Ooh ooh ah ah ah, nobody says the 'B' word" which, of course, is a rule.