A Google Webmaster Help thread has discussion where one webmaster is having the hardest time removing a URL from the Google index. He/she used the Google Remove URL Tool only to find out that even though Google reported back the URL was removed, two weeks later, it is still there.
Susan from the Google Webmaster Central team told the individual to try again and select "outdated or dead link" option in the form. This time, the user received an error that read:
You have already made this removal request.
Susan confirmed that this was some sort of bug. Susan said:
Congratulations, you may have found a bug.
Our team is looking into it. I can submit that URL for removal for you in the meantime, since it 404s.
I believe in this case, the page has finally been removed due to Susan taking manual action. I don't think this is a wide spread bug, but just a small bug.
Forum discussion at Google Webmaster Help.