I've seen, for at least 2 sites, where a keyword ranking was lost completely for a few days (from #4 to #650 - almost at the end of what is listed), and then regained. It has to be during the google dance (
http://www.fm-transmitter.com/google-dance/ ), when it's changing rankings. But this happened within the past few days, and the dance normally happens near the end of the month. So, fuck knows...