The polygraph test scenes had a few errors. Examiners always ask the subject a known false question to gauge a known deceptive answer. The examiner only asked questions where a known true or unknown answer was required. After Pinkney passed the test, Gillespie says they have handed Pinkney his case. As Tibbs noted, polygraphs are not admissible in any court so the results would not have benefited Pinkney.
The vehicle ID number Jamison reads to Parker was R7H3B90. That is too short for a post-1981 vehicle such as the Ford Taurus, which use 17 character numbers. Additionally, the plate shown had many more digits than the 7 that were read.