When the flying banshee smashes into Sharon and Jack's bathroom window Sharon has a white face mask on but when she goes down stairs her mask is gone.
During the scene with the spinning bed the man cranking the wheel above it falls through and lands on the bed with Miranda and Tony. They are all three on the bed as they slide down the stairs but when they reach the bottom and Plunkett ends up in the bed the man is gone.
The champagne levels in the glasses during Jack and Mary's romantic evening scene change at every shot.
The ghost of Mary Plunkett says she is praying to Saint Brigid to release her from her marriage vows. But marriage vows are "till death us do part" -- so Mary was released from her vows 200 years ago, when she died.
In three different shots wires used to "fly" people or objects are visible. First, when Jack Crawford (Steve Guttenberg) gets blown out the front door of Plunkett Castle (at around 34 minutes). Second, a few seconds later when the roof of the car gets blown off (at around 38 minutes) wires attached to it can be seen in the upper left corner of the screen. Third, when Malcolm (Martin Ferrero) gets hit by lightning (at around 13 minutes).
When Miranda's bed falls through the floor with her, Father Tony, and the other man on it, as the bed breaks through floor after floor, it's quite obvious the "people" on the bed are mannequins.
During the final dance sequence, Beverly D'Angelo accidentally knocks Daryl Hannah's hand making her spill her champagne on the floor. Daryl briefly makes a face before continuing the dance.
There's no explanation given as to why Mary and Martin would repeat the murder scene twice in one night, as we see when Jack stumbles upon them. Everywhere else in the film, Mary implies that she was killed once a night.
After the big wave from the giant squid knocks everyone over, and the pull the boy back out, Peter Gallagher is laughing uncontrollably and everyone else is terrified.