Expandable Foam
Otto Bayer was trying to invent synthetic thread to challenge DuPont's Nylon -- his own colleagues told him to throw the result away.
In 1937, German chemist Otto Bayer was working at IG Farben on the creation of a synthetic fiber. DuPont held the patent for the radically transformative Nylon, and the hunt for a clean alternative was on. Bayer's team's discovery of the diisocyanate polyaddition process led to the chemistry for polyurethane plastics, the precursor to the stuff in a can on hardware store shelves.
Colleagues were skeptical of Bayer's foaming polyurethane, but work continued to refine the underlying chemical reaction and patenting it, largely without convincing anyone what it was actually good for.
It took roughly fifteen more years before the foam reached commercial viability as an insulator, entering production in the early 1950s. Light enough to later help insulate spacecraft, today it's the default filling behind refrigerator walls and home insulation. Of far more significance was the underlying chemistry of polyurethanes, a versatile branch of plastics used in various forms today.
A byproduct of plastic research to rival Nylon, expandable foam was initially dismissed as worthless.