Continuity mistake: During the trial when Beverly is cross-examining Mrs. Ackerman, her neighbor, she asks if those were her scissors found sticking out of Mrs. Sterner's stomach. That's not where the scissors should have been found. After stabbing Mrs. Sterner with the scissors, Beverly pulled them out and threw them at the husband twice, both times they ended up sticking in the walls, and that is where the scissors would have been found when the police arrived.
Factual error: When Mr. Sutphin finds Beverly's things on Richard Speck, there's an envelope he mailed her, with his return address, which is wrong. It is listed as Stateville Correctional Center (which is where Speck served his time in real life): Joliet, Illinois 60434. However, SCC is located in Crest Hill, Illinois 60403.