I found this joke on a bookcrossing thread (posted by nwpassage):
A collector of rare books ran into an acquaintance who told him he had just thrown away an old Bible that he found in a dusty, old box. He happened to mention that Guten-somebody-or-other had printed it.
“Not Gutenberg?” gasped the collector.
“Yes, that was it!”
“You idiot! You’ve thrown away one of the first books ever printed. A copy recently sold at auction for half a million dollars!”
“Oh, I don’t think this book would have been worth anything close to that much,” replied the man. “It was scribbled all over in the margins by some clown named Martin Luther.”