The explanation is really pretty innocuous. We had an unaccounted-for edge case that cached paulphusion.de's pages under our root domain, posterous.com.

Like many software bugs, it was an edge case that we didn't foresee, but that happened extremely rarely. We've been running Varnish for the better part of a year, millions upon millions of requests and this was the first time something like this has ever happened.

Of course, none of that wipes the egg off of our face. ;)