What's the most game-changing discovery that got swept under the rug?S

Nobody expects total realism from science fiction or fantasy. But it's a big problem when a fantastic new device or ability is introduced — one that could change everything — and then it's never seen again. What's the most ridiculous potential game-changer that got totally ignored afterwards?

Please include a picture or clip of this miraculous discovery that got un-discovered. Thanks!

Top image: The Guardian of Forever from Star Trek