In other words, make things really hard or impossible to screw up on accident. (making things hard for people that are TRYING to screw them up is a different story).
Your camera battery doesn't fit in your camera if you try to install it backwards.
Your clothes dryer won't tumble if the door is open.
Your address is printed once, on the electric bill, and is visible through the envelope window, so no one gets mailed a stranger's bill.