Two stories to tell a lesson that has been learned time and time again - talk to your customers and validate your thesis before building.

• Masters of Scale podcast on Rent the Runway, then read Jennifer Hyman's own writeup. I especially recommend her analysis of the flash sale boom and bust of the 2010's (Groupon, Gilt).
• I wasted $40k on a fantastic startup idea, then read Hacker News responses
• The Mom Test by Rob Fitzpatrick, with summary