Let not the cobbler go beyond his last.

Meaning: Each person works best within their area of expertise; overstepping into unfamiliar domains risks poor results.

Origin: Pliny the Elder tells the story of the Greek painter Apelles, who took a shoemaker's criticism of a sandal in one of his paintings and corrected it. When the shoemaker went on to criticize the leg, Apelles told him not to judge above the sandal. The Latin ne sutor ultra crepidam became the rebuke for anyone offering opinions outside their expertise; the last is the foot-shaped block a shoemaker works on.