What is Communis Error Facit Jus?

Legal Definition
Common error makes law. 4 Inst 240; Noy, Max. p. 37, max. 27. Common error goeth for a law. Finch, Law, b. 1, c. 3, no. 54. Common error sometimes passes current as law. Broom, Max, 139, 140.
-- Black's Law Dictionary
Legal Definition
Common error makes law. See 154 Mass. 290, 13 L. R. A. 275, 28 N. E. 260.
-- Ballentine's Law Dictionary