Which innovative product was initially rejected by its own company's engineers as 'having no market value,' only to later become one of the most successful office products in history?

Throughout history, groundbreaking product innovations have transformed industries and everyday life. Some inventions were so revolutionary they created entirely new markets, while others disrupted existing ones. Behind many of these innovations are fascinating stories of persistence, vision, and sometimes even accidents. This trivia question focuses on a pivotal moment in product innovation history that exemplifies how a company's commitment to pushing boundaries can lead to extraordinary market success.

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Post-it Notes (3M)
Xerox photocopier
Apple iPod
Sony Walkman