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.
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: When Charles is looking out of the window, he pushes one slat of the blinds upwards, and then bird watches with his binoculars. When Lydia comes in and makes him jumps, he turns around, and this particular slat falls back downwards, into place. But in the very next shot this same slat is pushed in an upwards position again. When he sits back down again as Lydia leaves, the slat is back down again. (00:27:35)
Other mistake: When Adam and Barbara have their sheets on, trying to scare the mother, they walk out in failure and Lydia takes pictures of the ghosts - polaroid photos fall out (too quickly) and Lydia picks up one which is already developed - they take longer. (00:39:25)
Chosen answer: Because he can't say his own name, therefore the charades is the only way he can get her to figure out what his name is to say it.
Guy