If I search for
"Ruby off the rails" site:stackoverflow.com, I don't get Ruby off the rails in the first few pages. However, a separate question about hosting does, and there are links to the question I was searching for from the first page's hits.
Update: I tried some different searches to see if it was two titles having a similar name that was the problem. Instead, I tried How do I replace an executable with a mock executable in a test? site:stackoverflow.com, and found that that got no Stack Overflow hits. Removing the site: limitation got hits for copycat websites.
Another update: Searching for
"Is there a secure way to set up a mysql root password?" got only http://stackoverflow.com/sitemap.xml within Stack Overflow.