When Delia writes "Mauve" on the wall, the M is completely formed, but in the next shot there's a little bit missing from the M. Also it's possible to see rest of "Mauve" spraying (not perfectly cleaned from wall) when Delia and Otto enter that room.
When the Maitlands are scrambling to get back to the attic door and away from the sandworm, a far shot shows the white door hovering in space some distance above the sand. But in the next closeup, the Maitlands are almost even with the door while standing.
When Beetlejuice turns himself into a snake from the stairway railing, in one shot the railing is gone, in another the railing is back even though he's still the snake. Then in another shot the railing is gone again. Until finally he isn't the snake anymore.
When Charles is looking through his bird book, when he first opens the cover his ring is one solid gold band. Then the cameras turn away from his hand. Then when you see his hand a couple seconds later his ring has changed drastically.
After Otho climbs through the window, causing the blind and drape to fall down, we see him holding the crumpled up drape in a closeup of him and Delia. When the camera cuts back, the drape is nowhere to be seen.
Between the attic scene where Adam draws the door and the one where they return from seeing Juno there is a bench or table with tied up newspapers along the wall that Barbara was facing when Lydia comes up the stairs, despite no one being in the attic while they were gone (Charles reveals this by not knowing about the model later in the movie) the bench and newspapers have been moved.
In the calypso scene: When dancing the pack of cigarettes in by the one ashtray keeps moving from being flat to tilting on the ashtray to being on it's side, while they are dancing the cigarettes are shown only moments after being full length as being completely burned in the ashtrays. When the shrimp reach up to grab their faces some fall on the table, however each bowl only ever showed 5 shrimp during the scene and each hand still had 5 fingers.
After the Beetlejuice commercial when Adam is about to poke Lydia's key out of the attic door, the handle is still seen being jiggled on Adam's end, but in the next scene Lydia's hand is not on the knob.
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.
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.
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.
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.
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 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.
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.
While Lydia tries to open the attic door with the key, you see her on the outside letting go of the handle. As Adam picks up the screwdriver to poke the key back out you can still see the handle shaking as if she is trying to turn it.