Joel Spolsky discusses Five Why’s, a method of determining what went wrong in a system and an approach to preventing it in the future.
In an occupation where outages are high profile and we never like to do the same thing twice, we’d all be wise to heed Joel’s story and advice.
Down Not Across 
To be honest it seems silly that this story is based off an ISP that allowed a single switch failure to keep their whole operation down. If we were talking multiple NICs to multiple switches, and then still talking failure, it would have made a much better article and really stressed the whole 99.999999 point much better.