On July 20, 1973, Lee had been working on a film called Game of Death. While attending a dinner, he began to have headaches. He took a painkiller that he was allergic to, and died the next morning. At this time, the filming of Game of Death was only partially finished. The movie's producers decided to use the pieces of the film already shot, and combine them with new footage of other actors.
To explain this, the producers filmed at Bruce Lee's actual funeral, to write into the story that he faked his death. That way, all of the fill ins would have a logical reason to wear disguises, still maintaining the impression it was the same character. In addition, to make the death credible, they set up a scene with a cardboard cutout of Bruce Lee glued to a mirror. Even crazier, the producers made a SEQUEL starring Bruce Lee, using more unused footage.