When I first started playing guitar two of the first songs I ever wrote were very similar to Pearl Jam's "Do the Evolution" and Offsprings "Gone Away". I wrote my songs at least a year before those songs were released but they were so similar to what I wrote that I basically abandoned my songs because I knew they would be viewed as "rip offs".

My brother invented online FPS games. He came up with this idea around the time that Duke Nukem 3D came out as he was playing modem games with his friends and thought of making an online-only multiplayer FPS using that newfangled internet thing that was on the rise at the time. Essentially he created games like Destiny and Overwatch. At the time my stupid ass said that such an idea would never catch on because it has no single player component.
My uncle invented breakfast sandwichs when he was a kid (so this is in the 60's). He would take his entire breakfast and jam it into his crousant or toast or whatever bread-like substance he had at breakfast. Everyone made fun of him but these days every fast food place in the country sells the damn things.
I invented the dual tank toilet where you can do a half-flush to save water. And then weeks later went to a co-worker's house to see the same toilet I "invented" in their bathroom.