TITLE: Telling right from left
QUESTION [15 upvotes]: I know a lot of people, some of them mathematicians, who have trouble telling right from left. This can lead to problems when you are composing functions, for example. 
When did this seemingly innocuous confusion lead to wrong results being published? Here is an example from arithmetic geometry.

REPLY [7 votes]: 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 $\mathrm{H}$)”)