When it became clear that the company did not have a bright future I took all my skills over to a local brokerage that had just had an internal IT meltdown. It was like starting all over again on day one of my first job.
My most humbling moment turns out to have been political. At the brokerage I was asked if I wanted to be the IT Manager. I declined because I didn't feel I had the experience. This planted the seeds of my departure 5 years later as the hack they brought into the department felt increasingly threatened by my success and skills. You can't ignore office politics.
The second moment was more of a huge missed opportunity. One of the board members of the startup found out I was leaving and offered me a role at his other company. Honestly I didn't realize at the time what a compliment that actually was. I would have been one of the earliest hires of a major firm that is still going strong decades later. I learned to not conflate "niceness" with business acumen. This person had his own money on the line at our startup and had every right to press people on their success or lack thereof. I just thought he was a jerk at the time.
"Good judgement comes from experience; experience comes from bad judgement."