Visible crew/equipment: During the scene where Frank's ghost pals haunt Lucy and Ray's house, Ray begins hitting a moving Raggedy Anne doll that is near them on the ground. Look closely - behind the doll you can see a black bar that is mounted to the ground, causing the doll to jump and move. (00:12:55)
Visible crew/equipment: When Frank is in the freezer in the hospital laboratory, as Lucy and Agent Dammers fight over the door latch, the brief reflection of a crew member wearing red is visible in the large polished stainless steel hinge on the door. (01:14:10)
Visible crew/equipment: Outside the chapel, when Frank backs into the pedestal the statue begins to topple over. In the shot facing up, as the pedestal falls and Frank puts his arms out to catch the statue, the wires used in this stunt are perfectly visible. (01:35:05)
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.
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