The comment by Shnitz about never throwing away a piece of film that seems to be useless is valid. You never know what actually might be on that exposed film.
Linda Morabito worked on the "Navigation Team" at JPL (the Jet Propulsion Laboratory in Los Angeles) for the Galileo spacecraft that was going around Jupiter. The useless photographs that the Galileo spacecraft took that were overexposed were given by the scientists to the Navigation Team because the overexposure produced little pin pricks of light on the photographs from the stars in the background. The navigators used the stars to tell where Galileo was pointed, where it was going, and what course corrections by how much they should recommend making to achieve the scientific goals that had been announced.
One night when Linda Morabito was working alone at JPL on the useless overexposed photographs from Galileo, she noticed something on one of the photographs that also showed a part of the Jovian moon Io. Linda was not sure what it was, and there was no one around that she could discuss this with. She was not sure if she should call, or not call, and awaken one of the scientists to talk about this. She decided to wait until morning when everyone came to work.
You cannot imagine the reaction from the "scientists" that morning when Linda Morabito showed to them what she saw on the photograph. It is easiest to say that the scientists "erupted." On the horizon of Io was the faint image of an umbrella shape from a fountain on the surface of Io. Linda Morabito had discovered that Io is the most geologically active object in the Solar System. She is now part of the history of our exploration and discovery in space.
All because of a photograph that was the type that Shnitz accurately described as "useless."