Sutor, ne ultra crepidam

Sutor, ne ultra crepidam is a Latin expression meaning literally "Shoemaker, not beyond the shoe", used to warn people to avoid passing judgment beyond their expertise.
Its origin is set down in Pliny the Elder's "Naturalis Historia" [XXXV, 85 (Loeb IX, 323–325)] where he records that a shoemaker ("sutor") had approached the painter Apelles of Kos to point out a defect in the artist's rendition of a sandal ("crepida" from Greek "krepis"), which Apelles duly corrected. Encouraged by this, the shoemaker then began to enlarge on other defects he considered present in the painting, at which point Apelles advised him that "ne supra crepidam sutor iudicaret" (a shoemaker should not judge beyond the shoe), which advice, Pliny observed, had become a proverbial saying. The Renaissance interest both in painting and classical antiquity made the expression popular again.
A related English proverb is "A cobbler should stick to his last". The Russian language commonly uses variants of the phrase "Суди, дружок, не свыше сапога" ("Judge not, pal, above the boot"), after Alexander Pushkin's of the legend. In Spanish speaking countries there's also a related proverb, ""Zapatero, a tus zapatos" ("Shoemaker, [tend] to your shoes"").
The English essayist William Hazlitt is the first to have used in print a disparaging adjective "Ultra-Crepidarian", as he wrote a ferocious letter to William Gifford, the editor of "The Quarterly Review": "You have been well called an Ultra-Crepidarian critic". Occasionally the word ultracrepidarianism has been used later.
Karl Marx ridiculed the idea: “ 'Ne sutor ultra crepidam' – this nec plus ultra of handicraft wisdom became sheer nonsense, from the moment the watchmaker Watt invented the steam-engine, the barber
Arkwright the throstle, and the working-jeweller Fulton the steamship."