Revealing mistake: At the end of the movie, the main creature has the head of a human. However, in a number of shots you can see the actor's neck showing how they made the head "appear" to be attached to this creature.
From Beyond (1986)
1 mistake
Directed by: Stuart Gordon
Starring: Jeffrey Combs, Ken Foree, Barbara Crampton, Ted Sorel
Dr. Katherine McMichaels: If there is a statistical correlation between schizophrenia and the pineal gland, they may be feeling or seeing what we saw.
Buford 'Bubba' Brownlee: Well, what about the hard on I got? Is there a statistical correlation for that too?