Original: Owing to the film's title difference in America and the United Kingdom, all scenes that mention the stone had to be filmed twice, once with the actors saying "philosopher's" and the second with them saying "sorcerer's".
Simplified: Due to the film's title difference in America and the United Kingdom, all scenes that mention the stone had to be filmed twice, once with the actors saying "philosopher's" and the second with them saying "sorcerer's".