My own startup about five years ago (I've worked at several others, all different types of failures): Running out of money. We were bootstrapped, and the style of work we were doing had some decent capital costs. We failed for a "bargain" (less than 7K), but we had cut to the bone and deeper while trying that. Side jobs to make rent just increased stress and distracted from the business. Why did we run out of money?…
This I would say is my main learning point. My first startup failed because of inexperience and team implosion following incapability of generating revenue fast enough on one side, and attract favorable investment on the other. Failure could have been lighter on both me and the other members of the team, was it not for a self-immolation mindset probably driven by glorification of "startup lifestyle". Things fail. Most of the time. Assume failure. Optimize for resiliency. Never expect success as the natural outcome, but rather consider it will come collaterally to continuous reaction to ugly realities, and damn slowly. You can stumble upon a rocket ship, but it is not how businesses usually get to come to life.