The movie star, who played Blane McDonnagh in the cult John Hughes film, was convinced the project was over and signed up for a play, in which he had to appear bald.
But test audiences who saw an early version of the film prompted Hughes to reshoot the ending of the movie - because they hated the idea of Molly Ringwald's character ending up with Blane.
And McCarthy had to don a wig to reshoot the climax of the film.
Speaking to Hughes film class students at the University of Southern California last week (ends29Apr11), Ringwald urged them to go back and watch the end of the movie and pay attention to her co-star's hair.
According to EW.com she said, "That was a really bad wig."