The idiomatic expression "exception to the rule" refers to an instance or situation that deviates from a generalization, expectation, or standard. Full definition
The only exception to this rule of walking the dog in the square is if the dog starts to relieve himself. (austindogtraining.org)
The only exception to this rule happens for credit cards with promotional interest rate offers. (valuepenguin.com)
Of course, there are always exceptions to the rule, but here's why I often prefer to cultivate people internally and advance them when the time is right. (entrepreneur.com)