In July of 1846, James Challis of the Cambridge Observatory began a search for the eighth planet, what we know as Neptune. Disregarding the precise calculations by John Couch Adams, Challis missed his opportunity. When Neptune was discovered by two German astronomers in September, Challis looked back at his records. Sure enough, he had recorded it on August 4 and again on August 12 without recognizing it for a new planet.
Over two centuries earlier, on December 28, 1612, Galileo himself had observed Neptune through his telescope. He drew a diagram of its location, using it as a "background star," a fixed point for reference to Jupiter's position. Galileo Galilei, pioneering astronomer and discoverer of Jupiter's moons, apparently failed to recognize Neptune for a planet as well.
After Pluto was discovered in 1930, Clyde Tombaugh checked his earlier photographs. Pluto's image appeared on two plates taken in 1929, a month after he started his search and nearly a year before he finally found the planet. Of course, these plates were not of as good quality as the ones on which he eventually found Pluto.
Astronomers around the world began digging through their files of photographs. Pluto's image was found on plates taken at Belgium's Royal Observatory in 1927; Harvard in 1927; Yerkes Observatory in 1921; and several in Europe as early as 1908. Most ironic were the images of Pluto on plates taken at the Lowell Observatory in 1915, when Lowell was still alive, and those taken at the Mount Wilson Observatory in 1919 in its search for Pickering's Planet O.