What is a word that means "usually overlooked yet critical to results"?

single-word-requests

I'm describing topics for exam preparation. "The seldom reviewed but always encountered" topics on the exam."


If you really want a "single word", you could go for something like oft-ignored, oft-forgotten, oft-overlooked, but I seriously doubt there's anything (based on that construction or not) that would disambiguate between things students fail to revise for, as opposed to things the teachers fail to review. Or indeed, anything that significantly implies such things are always encountered after that lack of preparation.

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