From Nathaniel Popper's “Digital Gold : Bitcoin and the Inside Story of the Misfits and Millionaires Trying to Reinvent Money", this heavily edited (by me, for clarity) excerpt:

“In late July Gavin and Satoshi got an e-mail from ...a programmer from Germany going by the screen name ArtForz, ... had found a previously undiscovered weakness in the code that ...made it possible to spend Bitcoins in someone else’s wallet.” “For now, don’t call it the ‘1 RETURN’ bug to anyone who doesn’t already know about it,” Satoshi wrote to Gavin.”

What is this 1 RETURN bug? It's obviously been patched, but I'm interested because the book doesn't delve into technical details.

(FWIW, I highly recommend buying this book).