Comment by Georgelemental

https://en.wikipedia.org/wiki/Exception_that_proves_the_rule:

> "The exception that proves the rule" is a saying whose meaning is contested. Henry Watson Fowler's Modern English Usage identifies five ways in which the phrase has been used…

Personally, I use it in cases like:

- Rule: Don't do X, it's a bad idea.

- Exception: One time, someone with very special circumstances did X, and with a lot of finagling and effort they managed to make it work sort of OK.

Or:

- Rule: This fortress was an impregnable defensive position.

- Exception: In A.D. 1305, the fortress was taken, with great difficulty and many casualties, by an attacking army 100 times larger than the defending force.

Or:

- Rule: This river never overflows its banks.

- Exception: Once in history, on the day of the biggest rainstorm in 1000 years, the river is recorded to have overflowed its banks very slightly for a short time.

The exception proves the rule because the circumstances necessary for the exception to occur were themselves exceptional.