Had it happen with a Pentax SLR once. Was taking photos of flowers, and had just finished a roll of film. A friend asked me a botanical question, and thinking about the answer to his question interrupted and distracted me from the film-change process enough for me to forget that I had not yet rewound the film.
It certainly is a horrible feeling to open up a camera and see the unwound film looking straight back at you. Fortunately, most of the roll had been shot in that same area, so I was able to retrace my steps and retake most of what I had just ruined.