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].
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.