Revealing mistake: When Barbara and Adam are digging up Beetlejuice's grave, you can see that a piece of cardboard states "FRAGILE" or something in small print which is proportional to Barbara and Adam at that time. However, the lettering should be ten times larger if the cardboard was proportional to the actual model.
Revealing mistake: When Adam, who should have no reflection, walks out of the door to go retrace his steps, he is really clearly reflected in the big glass panel in the door. Then after Barbara pulls Adam from the SandWorm land, they walk back into the house through the same doors, and are both reflected in the glass panels of the door. So much for ghosts...
Revealing mistake: After the scene where Barbara's fingers catch fire, the Maitland's walk down the hall (and Barbara says "I'll make us some coffee), and in the picture hanging on the wall to the left of the screen, we see their reflections. They shouldn't have reflections, as proved by the mirror.
Revealing mistake: When we see that Barbara and Adam are hanging out of the window in order to hide from the Deitz's, it cuts to Beetlejuice yelling "Oh boy. You guys are really a couple of spooksters." In this shot of him shouting, he does not have any white makeup applied to him, nor does he have any of the green patches that run down his neck. In this shot he does not look his usual "dead" state, but looks normal. They obviously forgot to make him look "dead."