I don’t know about wrong results, but compare Godement’s Algèbre (1966, and still 1980):
on dit que l’ensemble $x\mathrm H$ est une classe à droite modulo $\mathrm H$
(translation: “we say that the set $x \mathrm{H}$ is a right class modulo $\mathrm{H}$”)
with everyone else, e.g. his associate Bourbaki (1970):
les ensembles $x\mathrm H$, qu'on appelle classes à gauche suivant $\,\mathrm H$ (ou modulo $\,\mathrm H$)
(translation: “the sets $x \mathrm{H}$, which are called left classes following $\mathrm{H}$ (or modulo $\mathrm{H}$)”)
