As far as I can tell, in the U.S., 49 USC 46317(a) makes flying without a proper certificate a crime punishable by fine and/or imprisonment. However, the FAA is a regulatory agency, which means it can only assess civil penalties (e.g., civil fines or revocation of certificates).
In practice I've heard (and would expect) the FAA gets quite exercised over "airmen" found to be carrying passengers for hire without current and proper certificates.
What I'm wondering is whether individuals have ever been, or are routinely, subject to any scrutiny or regulatory or prosecutorial action if they fly without certificates and don't commit any other egregious violations of FAA regulations? E.g., if you only fly registered aircraft for your own amusement, and you never break a serious airspace rule, but you never get an FAA certificate, could you expect to go your whole life without any trouble from the FAA? If not, what sort of trouble might you expect and how would you get into it?
And has anyone ever faced criminal prosecution for flying without proper certificates who did not also commit a gross violation of safety while operating an aircraft for hire, or some other felony (e.g., smuggling)?