Which of these products was originally designed for a completely different purpose than what made it famous, after its inventor failed in his original goal?

Many of history's greatest inventions and discoveries came after multiple failed attempts or were discovered accidentally while pursuing something else entirely. The journey of product development often includes unexpected twists and setbacks that ultimately lead to breakthroughs. This trivia question focuses on the product design process behind a famous invention that transformed from failure to worldwide success.

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Post-it Notes, which were created after 3M scientist Spencer Silver failed to develop a super-strong adhesive and instead accidentally created a weak, reusable one
The microwave oven, which was invented after Percy Spencer deliberately set out to create a cooking device using radar technology from WWII
WD-40 lubricant, which was the result of a planned project to create a water displacement formula on the 1st attempt
Velcro, which was intentionally developed by George de Mestral after carefully studying how to replicate the hooking mechanism of burrs