Here's a random example. When I search "false dichotomy" (even without quotes), I get one hit, and it happens to be something I said in a post in the BP on religion. Not too surprising as I am one who studies formal logic and has a tendency to use that language. However, I find it really hard to believe that I'm the ONLY person who's said "false dichotomy" and that it was only said once. I seem to vaguely recall Mike saying it at least once or something along those lines. Now, I'm not sure if anyone else said it, but I myself definitely said it more than once, as is evidenced by this post I found on taxing the wealthy:viewtopic.php?f=11&t=7362&start=45
See my second post, first sentence on that page. You'll find that it's not the same as the one result that the search produces. No doubt if it missed that one, it could be missing others.
This was just a random example that I just found 10 minutes ago too. I clicked 5 pages back in the BP, scrolled backward through the thread until I found a substantial post by me with a fairly uncommon word or phrase, searched it, and discovered it was not detected by the search engine.