http://blog.cloudflare.com/why-google-went-offline-today-and-a-bit-about
Situations like this are referred to in the industry as "route leakage", as the route has "leaked" past normal paths. This isn't an unprecedented event. Google previously suffered a similar outage when Pakistan was allegedly trying to censor a video on YouTube and the National ISP of Pakistan null routed the service's IP addresses. Unfortunately, they leaked the null route externally. Pakistan Telecom's upstream provider, PCCW, trusted what Pakistan Telecom's was sending them and the routes spread across the Internet. The effect was YouTube was knocked offline for around 2 hours.