At one point Betty Anne Waters Hilary Swank tells Kenny Sam Rockwell that it's a good thing Massachusetts doesn't have the death penalty or he could already be dead. This however is incorrect. Massachusetts still had the death penalty in 1983, when Kenny was convicted. It was abolished the following year in 1984.
In various scenes where Hilary Swank and Sam Rockwell's characters are meeting in the jail, the Sprite that Hilary drinks from has the new logo from 2005 consisting of two yellow and green "halves" forming an "S" lemon/lime design. This logo had not yet been created during the implied times of these scenes.
The goof item below may give away important plot points.
Errors in geography
When Kenny is released, there is a Boston police car outside the
courthouse. His release actually occurred at Middlesex County Superior Court,
which is in Cambridge. Boston is part of Suffolk County. In actuality, the scene was filmed in Detroit.