What was the original 'failed' adhesive discovery that eventually led to the invention of Post-it Notes?

Product design often involves accidental discoveries and persistent development. One of the most famous examples is the Post-it Note, which emerged from a failed experiment. This trivia question explores the fascinating journey from laboratory mistake to essential office supply, highlighting how innovation sometimes comes from unexpected places and requires both scientific knowledge and market insight to transform into successful products.

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A super-strong adhesive that was too permanent for paper applications
A pressure-sensitive adhesive that was considered too weak because it could be peeled off easily
A heat-activated glue that required too much energy to implement commercially
A water-soluble adhesive that dissolved when exposed to humidity