When I was thirteen or so, I got a job offer for independently inventing AJAX. Everywhere I looked at the time said you couldn't read data from the server using Javascript, but I realised that if you opened a new window to a separate page it could be done.

I didn't take it anywhere, and later (after people discovered xmlhttprequest) I learned that even at the time I could have used an iframe, which would have been nicer. I assume people were doing that before I made my discovery, but I don't know for sure.