The fibula and the safety pin
Walter Hunt patented the safety pin almost 160 years ago.
It looks an awful lot like a fibula, which, of course, is used to hold your toga shut.
My friend Kevin has one (not a toga, a fibula). An old one. He's very proud of it.
So, the question that Walter Hunt didn't ask is this, "Why should I bother patenting the safety pin? It's already been done. I mean, even John Belushi has a fibula."
Just about everything has a strike against it. It's either already been done or it's never been done. Ignore both conditions. Pushing an idea through the dip of acceptance is far more valuable than inventing something that's never existed... and then walking away from it.