It is well known by fans of the movie 'Jaws' that there were immense technical problems in getting the robotic shark to work. Indeed, this is the reason why the shark never even appears in the movie until well after the first hour. This actually turned out to be a blessing, for it can be argued that one of the main reasons the movie works so well is the build-up of suspense.
Anyway, Spielberg was extremely frustrated about the shark not working and wanted to change the entire movie so that instead of a killer shark, there would be a killer Snuggles. The studio was not very receptive to this idea. Nevertheless, he secretly shot some footage that has never been revealed.
This famous scene shows Chief Brody sitting at the beach, worried that another shark/Snuggles attack will occur. Notice the technique that is used of figures moving past the camera, which then wipes to a new shot. Brilliant!