What was the original failed product research that eventually led to the invention of Post-it Notes at 3M?

Some of the world's most useful products came from unexpected discoveries or 'happy accidents' in the lab. The journey from experimental failure to market success often involves innovative product design, marketing strategies, and brand storytelling. This trivia question explores one such famous invention that revolutionized office productivity and became a cultural icon despite its humble beginnings.

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An attempt to develop a super-strong adhesive that accidentally created an unusually weak one
A waterproof paper experiment that produced paper that could only hold temporary ink marks
A color-changing dye project that resulted in yellow paper that resisted fading
A heat-resistant coating test that unexpectedly created paper that was easy to remove without tearing