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)
Continuity mistake: When Mrs. Waterhouse walks into the livingroom and greets her three babies, there is a large area rug in front of the coffee table. However, as the babies fly towards her in the wide shot facing her, that area rug is gone! (00:22:55 - 00:23:50)
Continuity mistake: When Frank returns home from the Lynskeys, he parks his Volvo right beside the outdoor fuse box post, with the box facing beyond the trunk of the car. However, after Frank goes upstairs, in the next exterior aerial shot the Volvo is 10-15 ft. away from the fuse box post and the box now faces beyond the front of the car. (00:17:30 - 00:19:20)
Continuity mistake: At her house, Patricia runs away from the ghost and when she first enters the kitchen the countertops beside the sinks are completely bare. However, a few shots later there are neat stacks of china on the countertops, ready to fly off in the mayhem! (00:01:45)
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.
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.