LightSpeed Systems' content filter has blocked ServerFault.
This affects a tiny portion of the audience, myself included. LightSpeed provides network filtering for schools and government agencies. They have ServerFault classified as "Security.Proxy" because their bot found the [Proxy] tag. I submitted a request to reclassify it, but was apparently denied.
As of 3pm Eastern, they've stopped blocking it. I edited the question title and body to be more generic.