Is there a term for an instruction so obvious, anyone foolish enough to need it is too foolish to follow it?

I saw security cam footage of a guy in what looked like a sporting goods store. The guy picked up a knife, pulled out the blade, and stabbed himself. He then seemed surprised when he started bleeding. Someone in the comments said "There should've been a sign saying the knives were sharp." I think anyone dumb enough to STAB THEMSELVES not expecting to be injured isn't smart enough to listen when they are told DON'T STAB YOURSELF.