 Guillermo del Toro (qv) is famous for compiling books full of  notes and drawings about his ideas before turning them into films,  something he regards as essential to the process. He left years  worth of notes for this film in the back of a cab, and when he  discovered them missing, he thought it was the end of the project.  However, the cab driver found them and, realizing their importance,  tracked him down and returned them at great personal difficulty and  expense. Del Toro was convinced that this was a blessing and it made  him ever more determined to complete the film.