Thomas Lowry, 78, has confessed that he changed the "4" to a "5" on a pardon signed by Abraham Lincoln. The document was signed on April 14, 1864, but the alteration made it look like April 14, 1865. As a result, the document was assumed to be the last document Lincoln ever signed, on the day he was shot by John Wilkes Booth.
Altering the document is a crime but the statute of limitations has expired, so Lowry won't be prosecuted. He has, however, been banned from the National Archives.
You can read more in an NPR written by by Mark Memmott as well as in a video at http://www.youtube.com/watch?v=qKo8H9NN4nA or click on the image below.