Since boring crypto shouldn't have weird failure modes like this, I'm thinking this design is a big mistake?
EDIT: I think it was Instawallet and apparently while they had robots.txt set to prevent crawling, the theory was people typing their URL into Google (or Omnibar) would alert Google to the URL and it got into search results anyway.
I know that web-keys is based on the theory that since the fragment isn't sent by User-Agents in the Request-Uri that it's secure, but there are things that see the full URL which aren't conforming agents, and it just seems risky for any long-lived secret.