Revealing mistake: At the Fairwater Tuberculosis Sanatorium, when Agent Dammers shoots Frank in his left arm, the white wrapping around his arm, where the bullet holes appear, is visible under Frank's short sleeve. (01:36:05)
Audio problem: After Agent Dammers shoots Frank in the arm, in the close-up he shouts, "I don't want to shoot you in the back...I said turn around!" In the next close-up of Frank, Dammers repeats the latter, though nothing is heard. (01:36:20)
Revealing mistake: After Lucy discovers Mrs. Bradley in her bed, as she fights off and runs from Patricia, it is obvious that she wears knee pads for these shots.
Continuity mistake: At the cemetery, as Frank grabs Ray the ghost to pull him from his grave, the grave diggers are seen shoveling dirt into the half-filled hole. A few seconds later, Frank yanks Ray out and the grave is now completely filled and the grave diggers are gone.
Continuity mistake: The colours on the dial of the freezer Frank goes in changes between shots. When Lucy turns it down you can see the colours are light and dark blue. Later on in the wider shots and when she tries to open the door and Milton Dammers stops her, you can see it's blue and red.
Other mistake: When Frank first runs into Ray's ghost on the street, and they duck into an alleyway to talk, Ray accidentally calls Frank (Michael J. Fox) by his real name "Mike" instead of his character name "Frank." Ray says "I felt this vise-like grip squeezing my heart, and I couldn't breathe, Mike."
Visible crew/equipment: When Frank is walking down the street, after he walks into the pierced man in the grim reaper T-shirt (Peter Jackson in a cameo) when he walks across the street when he gets hit by the black hearse, the camera man, the camera and a few crewmembers are visible in the car door.
Continuity mistake: When the 3 "flying" babies crash into their mother, look at the one in the red clothes. In the wider shot it has a lot of dribble on its chin, but this vanishes in the closer shot.
Revealing mistake: When Frank, and later Ray, are shooting the water pistol, in the majority of the shots nothing is actually coming out when something is supposed to be.
Revealing mistake: A squib (a small explosive device) is seen on Frank's arm when Dammers shoots him.
Chosen answer: For the most part, the actors playing the ghosts were shot separately on blue-screens, and then comped into the scenes. (By removing the blue background through a process called "color keying", and placing them over the scenes) They were simply given a blueish glow and made semi-translucent to complete the effect. It was pretty cutting edge back in the 90's, but nowadays, it'd be very easy to make the exact same effect using a simple program like Adobe After Effects.
TedStixon