Revealing mistake: Adam and Barbara aren't supposed to have reflections, but when they enter the recreation of their home (just before they meet Juno), their reflections are shown perfectly in the glass display case that holds Delia's sculptures, behind them. (00:32:55)
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. (00:44:35)
Revealing mistake: When Barbara is moving the little plastic horse in front of the mirror to prove that she's invisible, the "reflection" of the horse is not synchronised with the actual horse. (00:09:10)
Revealing mistake: In the scene where Barbara is showing Adam that they have no reflections by moving the horse in front of the mirror, Adam's reflection can be seen in the windows in the background. (00:09:10)
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... (00:08:50)
Revealing mistake: In the scene when Beetlejuice is getting married, you can see the priest in the fireplace after he supposedly burns up. (01:21:15)
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. (00:08:30)
Revealing mistake: When Beetlejuice rises from the grave, and then flies after Adam and Barbra, for a split second, you can see a wire over one of his shoulders, but it is pretty hard to spot. (00:45:05)
Revealing mistake: When Delia and Otho go upstairs, Delia takes her can of spray and writes the word "mauve" on one of the walls. The faded marks of mauve spray paint are quite evident on the wall as she sprays the word, which reveals that this was not the first take of the scene. (00:16:00)
Revealing mistake: At the end when Lydia is being lifted into the air during the "Senora" song, if you look very carefully, you can see the wire lifting her up.
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: At the end, as Beetlejuice's head shrinks, the collar has a line around it, showing where the two images were superimposed.
Revealing mistake: When Beetlejuice is using "Zagnut" candy bar as the bait for the fly on his grave, the size of the "Zagnut" is not realistic - the fly should be much smaller than the "Zagnut" bar. (00:36:35)
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)
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." (00:56:40)
Revealing mistake: When the charred man is offering Adam a cigarette, you can see the hole in the headrest of the chair where the actor is sticking his head through. (00:31:49)
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.