During the final fight scene in the water between Hardy and the killer, there are two shots of a bald Bruce Willis, missing the hair (or wig) that he sports throughout the rest of the movie.
Jo isn't wearing a bra at the police ball; but, when she is undressed later, she is mysteriously immediately wearing one.
In the opening chase scene, the killer's car stops short at the police roadblock and a driver's side hubcap comes off and rolls forward. Later in the chase, both drivers' side hubcaps are still attached.
When Tom is taking the boat and Jo to go look for Chicanis, it's still broad daylight when they leave the boundary of their patrol area. It's dark when they get to the place Tom find Chicanis. However, when the kids find a floating body in the park, they arrive at the scene long before the regular police arrives.
When both cars crash at the end of the opening chase, the trunk of the killer's car flies open and is badly crumpled when it rolls. Later, we see two officers force the perfectly intact trunk open and a body falls out. The body would have come out in the crash when the trunk opened.
During the final fight, the underwater sequence shows the water as being crystal-clear. All three of the Pittsburgh rivers (Monongahela, Allegheny and Ohio) are very murky, due to barge traffic and residual pollution.
At the court sentencing, the bailiff announces the case as "State vs. Jimmy Detillo." Criminal cases in the Commonwealth of Pennsylvania are referred to as "Commonwealth vs. (defendant name)."
When Sarah Jessica Parker's character begins searching through Hardy's news clippings on his boat one of the headlines in the paper reads, "City Paralized ..." misspelling paralyzed.
When Tom Hardy arrives at the cabin, he quickly jumps off the boat without securing it to the dock. Later, the boat is still there, which is impossible on a flowing river.
After the car chase goes through the tunnel, Captain Hardy says, "We've got fire and two officers down in the southbound tubes, send an ambulance." The tubes are a nickname for the Liberty Tunnels and they are traveling through the Armstrong Tunnels.
The dead woman lying upside down in the water, blinks after being turned around.
Obvious stuntman for Tom in the "jumping river boat" shot.
When Jo is looking through the newspaper clippings, one headline reads, "City paralized with fear, another body found." The correct spelling, of course, is "paralyzed." Also, close inspection shows that the same text is repeated exactly between articles. For example, the two columns in the clipping entitled "Death toll rises in killing spree" are copied word-for-word in the "City paralized (sic) with fear" article and the "Nurse Paula Puglisi found dead" article.
Obvious stuntman for Robert Pastorelli when Tom kills Jimmy with the taser at the end of the water fight.
In the chase scene down the road with the big jumps, the added ramps are visible. They are painted to look like the road surface but are slightly lighter. Also to keep perspective correct, the sides are painted to match the grass and telephone poles next to the road. When the blue car jumps over the ramp closest to the camera, it visibly shakes and bounces up from the road surface.
A few times in the film Det. Eiler publicly derides Det. Hardy for violating the blue wall of silence, calling him "a rat" and "a prick," etc. On at least two occasions, he did it while reporters were present. They may or may not have heard him, but no cop in their right mind would be so reckless to flaunt their belief so publicly that cops are above the law. The offending cop could easily lose their job and open their precinct to internal and external review.
The Pittsburgh Penguins poster shown in the beginning of the movie, is a poster celebrating the 1992 Stanley Cup winning team (the 25-year badge clearly seen at the top of the poster commemorating 25 years since the team was formed). But, before this scene, the movie claims this is Pittsburgh in 1991 and clearly summer, a full year before this poster was available.
When Jo reaches for the case files inside Tom's house boat, you can clearly see a clothes pin (peg) holding back the shirt, probably due to it being another person standing in for her.
When Hardy is driving the boat the camera is reflected in his sunglasses.
In the tunnel in the opening chase scene, the killer spins out the police car. Just before the killer rams the police car there is a close-up of the two cars and you can see the camera crew on the right of the tunnel just beyond the barriers.
Polish Hill is only about two miles from downtown. The patrol run for the witness begins on the Allegheny near downtown in broad daylight, and ends in darkness.
The courthouse scene opens with an establishing exterior shot of the New York State Supreme Court on Center Street in New York City, not Pittsburgh, Pennsylvania where the film is set.
Nurse Paula Puglisi is said to have been working at Mercy Hospital. However, her badge reads: 'Magee Hospital.'
Before the chase, the dispatch reports a pursuit of a suspect in a 1989 Ford, but the Ford that was being chased was around 1985-1987.