Expandable Foam
Otto Bayer was trying to invent synthetic rubber during a wartime shortage -- his own colleagues told him to throw the result away.
In 1937, German chemist Otto Bayer was working at IG Farben on a problem with real wartime urgency: Europe was short on rubber, and someone needed to invent a substitute. One batch, contaminated without his knowledge, foamed up into a strange, structurally useless mess instead of behaving like rubber at all.
He showed it to his colleagues anyway. Their reaction was not encouraging -- they told him plainly they needed rubber, not Swiss cheese. Bayer kept working on the foam regardless, refining the underlying chemical reaction and patenting it, largely without convincing anyone what it was actually good for.
It took roughly ten more years before anyone figured out the punchline: that useless foam was one of the best insulating materials ever discovered, light enough to later help insulate spacecraft, and today it's the default filling behind refrigerator walls, house insulation, and the can of hardware-store sealant that expands the moment it hits open air.
Polyurethane foam was not designed as an insulator or sealant -- it was an unwanted accident during rubber research that Otto Bayer's own colleagues initially dismissed as worthless.