A patent may be found to be invalid for several reasons. An accused infringer will inevitably search the prior art in an effort to find pertinent references not considered by the Patent and Trademark Office when the patent was granted.
Prior art (anticipation, obviousness): Reexamination of a patent is a procedure whereby prosecution is reopened to allow the patent examiner to consider prior art not considered during the original prosecution. Uncertainty as to the validity of a patent, in light of newly discovered prior art, can often be resolved by reexamination.