In the scene where Quincy and his girlfriend Lee survey the area in Mexico where the first victim was killed by driving over a cliff, a panoramic view of the area beyond the cliff shows that it fronts on the ocean with only water visible. But when Quincy's is later thrown from his car as it is being pushed over the same cliff, a glimpse of the area in the distance shows a large city (probably Los Angeles, not Mexico) with no water in the scene.
The plane supposedly taking off for Mexico is the "City of Everett," the Boeing 747 prototype. An airline would not be flying this plane.
When Quincy is talking to Monahan about the alleged perpetrator the reaction shot of Monahan is printed backward as his hair is parted the wrong way.
When the patient heart stops the nurse orders an injection. A nurse wouldn't be allowed to do this.