Which non-stick material was discovered by accident?

Teflon. In 1938 Roy Plunkett, working at DuPont on refrigerant gases, opened a cylinder that should have contained tetrafluoroethylene and found nothing came out - yet the cylinder still weighed what it had when full. He sawed it open and found the inside coated with a waxy white solid: the gas had polymerised on its own.

It turned out to be almost chemically inert, with the lowest friction of any known solid. Its first serious use was in the Manhattan Project, gasketing equipment handling uranium hexafluoride, one of the most corrosive substances then in industrial use. Cookware came much later, after a French engineer's wife suggested he try it on a frying pan.

The awkward part is not the coating but the manufacturing. PFOA, long used to make it, belongs to a family of fluorinated compounds so stable they are called forever chemicals; they do not break down, they are now detectable in the blood of essentially everyone, and DuPont's own internal knowledge of the health effects became the subject of very large litigation. The finished pan is inert. Making it was not.