Continuity mistake: At the beginning of the movie when Romy reunites with Heather Moony, Heather tells Romy that she invented the cigarette which she is smoking and takes a puff to show how fast it burns down, and the cigarette burns to the butt. But in the next scene her cigarette is still to its half-way point.
Continuity mistake: In the reunion scene Kristy Masters, the A group leader, is making the speech of where everyone has been for the last ten years. When they show Heather Moony, if you look closely at her dress it appears to be that she spilled her drink on her dress already, later after Kristy talks about the football player Heather drools on her dress and the earlier spot where she spilled has disappeared.
Factual error: In the prom scene in which Romy and Michelle wear Madonna costumes, Romy is wearing her hair up in a distinctive ponytail assumingly inspired by the famous ponytail Madonna sported in her Blond Ambition World Tour. However, this scene is supposed to be taking place in around 1987 and Madonna did not wear that ponytail until 1990.