What's one thing you definitely shouldn't do when starting a startup?

Overengineering. A startup should be as simple as possible at the beginning, from product to sales. Complex systems are not needed at the beginning.

Working in isolation instead of spending time with users and partners.

Definitely don't try to do everything yourself. Outsource/delegate tasks that aren't your core strengths so you can focus on the most important things. Also avoid spending too much time perfecting everything - better to launch something simple and iterate quickly based on real user feedback vs getting stuck in analysis paralysis mode.

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Being too confident and satisfied with the project to listen to other people’s advice or criticism.