For a long time, astronomers believed that there must have been a planet lurking in the gap between Mars and Jupiter. What they found was Ceres, and this object's story is one of scientific cattiness and our ever-evolving understanding of space.
Artist's view of bright spots on Ceres imaged by the Dawn spacecraft. ESO/L.Calçada/NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/Steve Albers/N. Risinger (skysurvey.org) ( Creative Commons Attribution 4.0 International License)