My friend in China says it isn't a 404, the connection is just RST'ed to both sides and dropped. Simple, and effective. Your browser should display a "connection reset by peer" message or something to such an effect.

In Syria, however, you do get an error page saying the page was blocked. Facebook, for example, is blocked in Syria (or at least, it was when I was there in 2009 - of course, I had a VPN to Ireland, so it didn't affect me at all).