Four planets were found orbiting the star HR 8799 in 2008. However, observations of the star taken in 1998 were found to have three of those planets in them, hidden by the glare of the star! Improved techniques in software and analysis revealed the planets, buried in the star's glare.
Original blog post: Exoplanets seen by Hubble in 1998 finally revealed
Image credit: NASA, ESA, and R. Soummer (STScI)