Best horror movie mistakes of 1989
Revealing mistake: In the beginning when Michael crawls out of the mine shaft that was just bombed he floats downstream. You see that he floats up to the old man's dock. If you look at him you can clearly see that he is wearing a black lifejacket. The filmmakers didn't even try to hide it under his suit, it is clearly visible on the outside of his jumpsuit.
Continuity mistake: There is no knife on the table when the wife enters the kitchen at the very end, however, she walks straight to Louis, hugs him and then magically a knife appears at the edge of the table which she picks up and uses to stab him (implied - the movie actually ends at that point).
Continuity mistake: In the scene where Jonathan finds Cooper in the bathroom possessed, Cooper attacks John with a knife in his left hand. As Cooper lunges at him, John slams the door, but Cooper's left hand crashes through with little problem. But, there's no knife. The knife is now in Cooper's right hand.