While following/watching Eliot toss the briefcase in the dumpster, the license plate on Vince's car changes styles between close and wide shots.
When Harry takes the pictures of Elliot and Helen from inside the vent, he is at angle far away from them. When he shows the pictures to Elliot later, the pictures show that they were taken from a closer point of view and from the side.
When Harry Crumb is undercover as the vice president of quality control at the spa, he smells the mud and gets some on his nose. A few seconds later, the mud is gone and his nose is clean.
When Harry first eyes the paper shredder in his new office, the collection bag is empty. After feeding a small card into the machine, and getting his necktie caught in it, the bag is suddenly full of paper scraps.
As the plane is taxiing from the gate the pilot announces that they will be flying at 32,000 ft. In 1989 that altitude was unusable. Above 29,000 ft altitudes were assigned in 2,000 ft increments, so after 29,000 you went to 31,000 then 33,000 and so on.
While speaking with Mrs. MacIntyre, oranges fall from the tree in Harry's office. When the first one falls, it would appear that a crew member's hand briefly appears pushing the orange off the table, but watching the scene in slow motion/frame advance reveals that the "hand" is actually the leaves of the orange.
When Elliot is tied up in the hanging chair and Harry hits him with the door, Eliot swings around and his feet hit one of the equipment racks, causing the rack to wobble as if it were just a hollow shell.
appears during the chloroforming/kidnapping scene that the one of the main victim was a second take. if you look closely when the bowl of mud is knocked over quite a bit already on the shoe but none dropping from bowl when on way down.
As Harry falls out of the vent and onto the apartment manager, knocking him out, the side of the apartment manager's face is different, meaning a stunt double was used.
When Harry drives off in the green car from the rental agency in Los Angeles, it clearly has no windshield wipers.
The ransom money is paid in $1,000 bills, which went out of circulation in 1969. While $1,000 bills were still available in 1989, they were very rare and cost more than $1,000. No one would've paid a $10 million ransom in $1,000 bills at that time.
When Nikki calls out to her father to let him know that Harry arrived at their house, her mouth is saying something other than "Hey, dad!"
Right after Harry Crumb falls out of the air conditioning vent, his camera falls out. A reflection of human hands can be seen in the metal vent, dropping the camera.
During the car chase, just after Harry's car breaks free, camera mounting equipment is visible on the driver's side of Vince and Helen's car.
When Detective Casey begins following the ransom money, camera and crew are reflected in the vehicle windows as he drives away.
When Vince and Helen drive towards the dumpster where Elliot parked, part of the camera crew is reflected in the rear view mirror right before Vince's face comes into view.
Vince would never have been able to get a handgun through the metal detector at the airport; let alone the fact that when he confronts Eliot and Helen as they embrace, and holds Eliot at gunpoint, there is not a single person around to see him pull a gun. Airports are never that deserted.