Exception That Proves the Rule

The second part of Cicero’s phrase, “in casibus non exceptis” or “in cases not excepted,” is almost always missing from modern uses of the statement that “the exception proves the rule,” which may contribute to frequent confusion and misuse of the phrase. In this example, saying “the exception proves the rule” is literally incorrect, but it is used to draw attention to the rarity of the exception, and to establish the status of the village prior to the exceptional event. The first speaker in this example has confused the meaning of the phrase, apparently believing that any exception to any rule “proves” the rule true; in this case, the notion that “the exception proves the rule in cases not excepted” is neither implied nor understood by the speaker.

Source: en.wikipedia.org