Revealing mistake: When the attorney Ms. Lawlor is being 'hijacked' by the ghost (we only hear his voice) Grubeck in her car, it is very obvious that a person in a black ski mask and black clothes is driving the car.
Plot hole: During the scene in Paula's frontroom right after Spike walks in and confronts the astral Mr. Grubeck harassing Robin, he takes a fireplace tool and swings at Grubeck and it naturally goes right through him, to which Grubeck replies, "You can't touch me, but I can touch you." However, just a minute earlier before Spikes arrival, Grubeck had Robin pinned against Paula's refridgerator and Robin physically pushed Grubeck away from her.
Continuity mistake: Towards the end of the film, Spike tells Robin he had learned, that in order to stop Mr. Grubeck's astral body from killing, they have to kill the physical body first. But at the very end of the film, Spike, who had been run down just five minutes earlier on his bike and killed by the astral-projected Mr. Grubeck driving a semi, shows up in astral form to push the real Mr. Grubeck over the mountain, in turn killing him. This obviously contradicts the entire theory.
Plot hole: At the end of the film, Spike goes to the outside of the jailhouse where the real Mr. Grubeck was being held and "challenges" the astral Mr. Grubeck to come out and chase him. The astral Mr. Grubeck agrees, and the scene turns into the astral Mr. Grubeck in a semi chasing down Spike on his bike. Spike, now about a mile ahead of the semi, turns around to chase the semi head on and throw a stick of dynamite at the astral Mr. Grubeck. Spike does blow up the semi, but considering this was the astral Mr. Grubeck, and Spike fully knew he could do no damage to him in astral form, Spike's actions are contradicting and pointless.