Veteran game designer Damion Schubert put up an interesting post to his personal site discussing some of the unintended consequences of design choices. Just as the seemingly simple decision to allow objects to have collision properties resulted in Ultima Online towers being robbed, design elements in other fields can have unexpected results.
Schubert points out the dangers of the Prius as a perfect example of these unexpected results. The fact that the hybrid vehicle is incredibly quiet probably didn't seem like a big deal to the Toyota designers. In fact, they may have seen that as a plus. A plus for everyone except those without sight, of course. The result is that the vehicle's designers are now considering how to add more auditory feedback to the vehicle. Check out the post and the attendant comments for some additional MMO design elements that seem, in 20/20 hindsight, kind of obvious.