Tristan McLeay wrote:

> The expression is 'the exception that proves the rule', and harkens back
> to an older meaning of 'prove' and 'proof' of 'test'. Though along with
> the changed definition of 'prove', the saying's been misanalysed as
> meaning what you said.

As others have pointed out, this is wrong, although it's a common

The original meaning is that if there's a legal principle which
states, for example, that it is permissable to throw paper aeroplanes
in the classroom between 10:00 and 10:30 am, then this is evidence
that there's an implicit legal principle to the effect that paper
aeroplanes may not be thrown in the classroom when the exception is
not applicable. (I've chosen a silly example because that's more fun.)