If you've ever fixed any kind of machinery, you know that a device that's exposed to the elements is incredibly difficult to maintain. A washing machine or the underside of a car gets grungy, fast.
On the other hand, the dryest, cleanest environment of all is the digital one. Code stays code. If it works today, it's probably going to work tomorrow.
The wettest, weirdest environment is human interaction. Whatever we build gets misunderstood, corroded and chronic, and it happens quickly and in unpredictable ways. That's one reason why the web is so fascinating–it's a collision between the analytic world of code and wet world of people.
No software design survives a collision with the user.