A mechanical pencil follows DOTW (Do One Thing Well). There is only one way to get lead out, and only one option to try. However it breaks down when the lead runs out. How many times have you shaken a pencil to see if lead is still in it?

Better: If, once the lead ran out, a bright red warning piece of plastic came out of the tip clearly indicating the error state.

A simple interface that ‘Does One Thing Well’ is highly desirable and guessable. Yet you must take into account all 3 states of an interface: Empty, Full, and Error.

If you have a clear picture of all three states when you design then so will your users.