Door handles are the same way. If there's a handle, it implies "pull". If there's a plate, it implies "push". This is called an affordance. The design of the thing affords you the opportunity to do the right thing naturally. That's what tonight's Portland Perl Mongers meeting was about.
I started thinking about this and wondering what is poorly designed and how we can improve upon it. The thought that came to mind was the condom. How many times have you been in a dark room and realized you were trying to put the condom on the wrong way? You feel stupid and you have to throw away the condom (if you think you don't have to throw it away, you don't know enough about STDs).
Here's the design challenge that would make someone rich: how would you design a condom so that you can always put in on correctly even if the lights are off? One person suggested flavoring one side.
My idea involved a tack.