A while back whilst trawling through my referrer logs, I noticed a hit from MSN search with the string "free downloads of ms-dos"; I checked it out, and I was number 8 in the rankings for that phrase - the words "free", "downloads" and "MS-DOS" appeared somewhere on various pages, but it was purely coincidental.
Anyway, being the charitable fellow I am (and having recently stumbled across an old version of MS-DOS on the MSFT website whilst looking for QBASIC (don't ask)) I figured what the heck, and added a link with that exact phrase, pointing at the relevant MSFT page. A week later, and searching for "free downloads of ms-dos" yields, well, not me.
So, what gives? How come when I just happened to have the search terms at random points across my site I scored highly, but now I have the exact phrase I'm not there any more? I don't care about this specific instance, but it seems to be a common thing these days...