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 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.
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 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.
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.
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.
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 tunnels that the car chase travels through, the Armstrong Tunnels, are two separate tunnels, each traveling in one direction. They would not have faced oncoming traffic while traveling through those tunnels.
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.