Let me put it in realistic terms, once every year or so!!
They went down Aug 2007 for more than 4 hours!!!
Believe me, my life on earth depends a lot at this time on this company and the way they do business, should I consider "No Single Point of Failure" on my career as well?
The August 2007 was attributed to a power loss. The August 4 2009 was attributed to "human error" that caused the "BGP tables on the Internet to drop a route that resolved to cisco.com - 188.8.131.52/24".
I hope that "human" still has his job.