Continuity mistake: Pooter's hair was cut in the scene where they sneaked out of class to go to the zoo, but when they got to the zoo his hair was grown out.
Factual error: In the opening scene, the guys hop a ride on the back of a CTA bus. That bus was a darker green with windows that would open and was true to the time period of the sixties A lighter lime green bus passed them from across the street. That bus had windows that did not open, because that's when the CTA introduced air-conditioned buses. Those buses rolled out in the early seventies, much later than the time period portrayed in the movie.