Reminds me about a big site in (www.bilddagboken.se)
Its a site where people can register and post pictureblogs. If I register as ryan the adress to my page would be ryan.bilddagboken.se.
After YEARS of running the place (around 800.000 users) someone registered the username "www". This resulted in all visitors that typed www.bilddagboken.se (or used a bookmark) was directly redirected to the user "www" page. His/Hers page had thousandths of angry comments accusing him/her of hacking bilddagboken.se
Every once in a while, signing into Twitter I get redirected to 'twitter.com/sessions' and instead of it handling my log in, it's an actual Twitter account which is inactive. Pretty odd. I think they finally fixed it...
They fixed it, btw www.paulphusion.de was still pointing to his old address at 18.104.22.168. I don't know what he did, but it seems that he didn't follow what posterous suggested (adding an A record to its domain pointing at posterous). Sadly i wasn't quick enough to collect more data about posterous.com...
Waiting for the official post, i'd say that Paul misconfigured something that they didn't test for that particular input.
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. ;)