Ne supra crepidam sutor iudicaret. Let the cobbler stick to his last.

-Apelles   4c
Quoted in Pliny Naturalis Historia, 35.36.85.  A cobbler who had criticized  Apelles' way of rendering a sandal in one of his paintings, proceeded to criticize the rest of the painting as well. The expression became proverbial.

For should thesoul of a prince enterand informthebody of a cobbler, as soon as deserted by his own soul, everyone sees he would be the same person with the prince, accountable only for the prince's actions; but who would say it was the same man?

-Locke,John
  Essay Concerning Human Understanding, bk.2, pt.27, section15.