The point here is that when you know that you're living with the Chaos Monkey, your systems become very fault tolerant. Living with a monkey does that.

But even you don't embrace the concept, the Chaos Monkey is likely going to become a uninvited house guest at some point in time. In StackOverflow's case, they bought a mainstream server with a mainstream OS, and discovered that that server came with a monkey.

Think of it another way. My brother and sister in law never worried about the failure characteristics of dinner plates, so they had lots of nice stuff. Then they had a baby. All of the sudden, falling plates and glasses became something that they had to think about.