My favorite variation on this move is the classic Castillo v. Texas where an adult walked into a section that said "No One Under 18 Allowed Past This Point," bought a book that said "Absolutely Not For Children," and got the clerk arrested for trying to sell adult comics to kids.
About a few years ago when new manga in Weekly Shounen Jump were constantly getting published to fill in the void left by the ends of long-running series, only to wind up being canceled early to the point where people joked about "the U19 Club," I flirted with an idea where the MC is isekai'd to...