It’s called moviemistakes.com. One entry from the boo-boos in the movie “Gangs of New York.”
Factual error: In one scene where Daniel Day Lewis is in the Mayor’s office, the Mayor is sitting behind his desk and holding a bunch of papers, and on the upper right hand corner of the papers is a paper clip. Paperclips were not invented until 1899.
Thanks to J.D. Lasica for the pointer to the site, and to a Newsweek story in which “Lord of the Rings” director Peter Jackson is taken on a nit-by-nit tour of his movies.