The idiomatic expression "to be an exception to the rule" means that someone or something is not following a general pattern, but rather stands out as unique or different. Full definition
There are exceptions to this rule as there are to every other, but these facts generally hold in most cases. (lactationconsultantresources.com)
In this case there's an exception to the rule of a temperature increase beginning around 1 / 10th of our sea level pressure. (principia-scientific.org)
But there are exceptions to this rule as well, and you can learn all about them in this article. (homebuyinginstitute.com)