odd number, faux pas
Mais aussi malheureux que vous soyez, faites un impair et vous êtes fichu.
But no matter how unhappy you are, make one mistake and you're done for.
— Stalker (1979)
I made a serious blunder.
Il devait avoir le trac, mais il n'a commis aucun impair.
He must have been nervous, but he didn't put a foot wrong.
— Titanic (1997)
Il a commis un impair quand il a demandé si elle était enceinte.
He made a mistake when he asked if she was pregnant.