The LA Times newspaper says it's thought someone used a digital camcorder pointed at the screen.
Million could see it before release
Experts say up to a million people will have seen the film before it is even released, which has never happened before.
Bootleg copies of films have been put online before but the movie industry's worried it's happening on a much larger scale.
A film can be downloaded and then copied onto a recordable DVD found in many home computers.