C'est l'exception qui confirme la règle

Literally: It's the exception that confirms the rule

Example: “Il pleut toujours en Bretagne, mais cette semaine il a fait grand soleil : c'est l'exception qui confirme la règle.” — It always rains in Brittany, but this week it was sunny every day: the exception that proves the rule.

English equivalent: The exception that proves the rule

Origin: From a Latin legal maxim, exceptio probat regulam in casibus non exceptis (in French legal tradition exceptio firmat regulam): "the exception confirms the rule in the cases not excepted." The idea goes back to Cicero's speech Pro Balbo (56 BC). Balbus, a Spaniard from Gades, had been made a Roman citizen, and his accusers pointed to treaties that barred certain peoples from citizenship. Cicero answered that a stated exception implies a rule for every case it does not mention: where no exception is made, the thing is allowed. The maxim in its Latin form is later, probably 17th century. Its original sense was precise, but in everyday French, as in English, the phrase is mostly used to wave away a case that does not fit, with the second half of the Latin forgotten.