Which failed experiment led to the invention of Post-it Notes at 3M in the 1970s?

Some of the world's most successful products emerged from unexpected discoveries or failed experiments. The journey from laboratory accident to market phenomenon often reveals fascinating insights about innovation and product development strategy. This trivia question explores one such iconic invention that transformed everyday organization and communication through a seemingly simple design solution.

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An attempt to create a super-strong adhesive that accidentally produced an unusually weak, pressure-sensitive adhesive
A paper bleaching process that resulted in yellow paper squares with unexpectedly sticky edges
A test for heat-resistant bookmarks that revealed paper could hold temporary adhesive
A manufacturing error that applied glue to the wrong side of transparent tape, creating removable notes