Sometimes designers and builders of a system make decisions about tradeoffs between availability and scaling and error recovery that they don't understand until after the system is in the field. Any sufficiently complex system becomes vulnerable to unexpected interactions between subsystems especially when one or more subsystems fails catastrophically.
http://en.wikipedia.org/wiki/System_accident has more on the topic, but you do want to read Perrow's book
http://books.google.com/books?id=VC5hYoMw4N0C&printsec=f... Normal Accidents.