Continuity mistake: There is a scene where Molly Ringwald is confronting her father about lying about seeing a counsellor. His hair is sweaty and messed up a bit and then it changes position to Molly Ringwald and back to her father. Then you will notice that his hair is combed.
Continuity mistake: In outdoor scenes outside of the record shop (mall area), extras are seen on different days in the same outfits.
Continuity mistake: At the end, at the prom, Andrew McCarthy's hair is clearly much shorter (and combed really weird!) than in the rest of the movie. Sure you could argue, "maybe he got a haircut for the prom". But a couple of scenes earlier, it shows everyone getting ready for the prom, including him, and his hair is the same as it is all through the movie, up until the prom scene. [One contributor explains: Andrew McCarthy and Duckie are both wearing wigs. I know this because my father made the wigs. The original ending had Molly Ringwald ending up with Duckie. They wanted to change the ending so that she would end up with McCarthy. In the meantime, he had gotten a crew-cut to appear in a play or movie about soldiers. I don't know what was up w/ Duckie's hair, but the last scene has him in a wig,too].
Continuity mistake: During the scene where Andie and Blaine are talking in the "freaks" courtyard, you can see the extra with the black-and-white leggings in the background of each shot. She's behind Blaine, then Andie, then back again instantly throughout the whole conversation.
Continuity mistake: In the scene where Andie and Blaine are talking in their school's courtyard, Andie's shades are in her hand in one shot but automatically appear on her head in the next.
Deliberate mistake: In the prom scene at the end, there is a shot of a couple dancing with his hand on her rear-end. A counselor comes over and hits the boy's hand to make him move it. He complies, they kiss, and he puts his hand back when the counselor leaves. The camera cuts away to a different shot, and then returns to an over-head wide shot of the crowd. Down in the lower left-hand corner you can see the exact scene with the couple and counselor being filmed.