Darkness, this is exactly what happened to me. I rapidly canceled a search, refined my terms, and searched again. I was explaining it to a friend tonight and tumbled to the fact that it's probably just a matter of a crappy algorithm (doesn't take many searches to trigger it = crappy) designed to prevent DoS attacks or the like. It's only happened a few times, but specifying a site to search seems to be a contributing factor, too.
"I'll tell you about my mother."
On the plus side, at least it doesn't ask me to do a Voight-Kampff test.
"I'll tell you about my mother."