The USPTO has recently added to its patent database approximately 500 patents issued from 1790-1836 that were lost for 175 years. These documents were among the approximately 10,000 patents destroyed when the Patent Office, which was located in Blodgett's Hotel in downtown DC, burned down on the night of December 15, 1836. Shortly after the fire, Congress appropriated money to restore the Patent Office files and copies of about 2,500 patents were eventually obtained from inventors.
Apparently, USPTO staff retrieved these 500 patents from circa 1970s microfilm located in the Public Search Facility, but the location of the original documents is unknown. Lost patents do turn up from time to time. In 2004, two patent attorneys discovered 14 missing patents in the library at Dartmouth College.