In looking over the specs for the backend, my guess is there is an IP based throttling mechanism to limit the number of requests by IP. if either you had bad DNS service (someone mucked up the records for mathoverflow.net server for you), that might make the serverlook unavaialable to you; if your IP (or your proxy's IP?) had been sending a lot of HTTP requests, the throttlingmechanism may have denied you temporarily. I don't know how it would look (ideally they would give you an alternate web page), but it might look like the server was down for you.
Gerhard "Or I Could Be Mistaken" Paseman, 2011.10.20
]]>