pour un point (or, faute d'un point) martin perdit son âne

This is said of a person who loses something valuable through a trifle. The Abbey of Asello (Latin asellus = little ass) was taken from the Abbot Martin on account of his punctuation of a sentence over the gateway. Instead of: Porta patens esto, nulli claudaris honesto (Gate be open, and be closed to no honest man), he punctuated: Porta patens esto nulli, claudaris honesto (Gate, be open to none, be closed to an honest man). His successor corrected the mistake, and added: Uno pro puncto caruit Martinus Asello.