Caution against thinking yourself an expert

From Dan Galvin's Thought For The Day mailing list:
  ... no one ever considers himself expert if he really knows his job.  A man who knows a job sees so much more to be done than he has done, that he is always pressing forward and never gives up an instant of thought to how good and how efficient he is. Thinking always ahead, thinking always of trying to do more, brings a state of mind in which nothing is impossible. The moment one gets into the "expert" state of mind a great number of things become impossible.
                 -Henry Ford Sr.

This is a good lesson to teach our children.