What if I accidentally contributed a little bit too much code to the Linux kernel? What if that code had been in the last 10 releases? What about the users and distributions running those versions?
I'm not sure the answer is, or should be, as simple as "derp - delete immediately, this was never meant to be Open Source".
That said, if the code was stolen, and then published under an OS licence - it's not OS. The "publisher" never had the rights to make it OS, so the "contract" is null and void...