Raiders of the Lost Patents
In 1836 a temporary building belonging to the United States Patent Office was consumed by fire. In it were all the records of the office, including the roughly 10,000 patents that had been issued since the Office was created in 1790. Most of the patents have still not been recovered, but two interested attorneys just tracked down two of them.
Oddly the New York Times article I'm linking to (you can use "songdog.net" as username and password if you don't have a free account) reports both July and December 1836 as the month of the fire.