The phrase "occasional exception" means that something usually happens a certain way, but sometimes there are instances where it happens differently. Full definition
The movie tended toward an amber / yellow palette, with occasional exceptions. (dvdmg.com)
While I agree wholeheartedly with the attitude that «it's not about you» there are occasional exceptions where more personal information is acceptable in the cover letter. (squawkfox.com)
Let's let Wikipedia explain: «Flexitarianism is a term used in the United States to describe the practice of eating mainly vegetarian food, but making occasional exceptions for social, pragmatic, cultural, or nutritional reasons.» (newscientist.com)