Solarization is a phenomenon in which the tonal values of an image are wholly or partially reversed as a result of extreme overexposure during the developing process. By re-exposure dark areas are turned light. The result is a combination of the negative and positive image.
Surely the invention of solarization was caused accidentally. This is one of the stories of its discovery:
Man Ray (1890-1976) and Lee Miller (1907-1977) worked in the darkroom when Lee Miller suddenly felt something on her feet. Shocked she turned on the light, thus exposing the photographic paper to light. When the photograph was finally developed, the effect became visible.