I wrote an article detailing an idea I had back during web 1.0. I thought it was clever and maybe some people would find it interesting. Few years later a SaaS popped up selling the same idea as a service. It eventually became a pretty commonly followed pattern for a bunch of huge companies in the same domain.
I have no idea if I was literally the first person to have this idea or if anyone who launched one of these businesses read my article. I definitely didn't understand how much commerical value there was or even considered making a business out of it. I blame nobody but myself for missing an opportunity if there even was one.
I did get like $200 for writing it which was nice.