I think for me, and for many founders up maybe even to $20m ARR or so, it’s losing key people you really needed in senior positions.
Other lows are tough. Losing key customers. Blowing deals. A bad launch. Not growing fast enough. Pulled term sheets. Being almost out of money. Being out of money. Your partners competing with you.
Those lows are hard. But with a great team, you can often find a way out of it. And importantly, you have the resources to do so.
But until you have redundancy in your start-up at all key levels, you are always a tough Lost VP away from not being able to make progress in a key area.
The earlier you can afford to build in redundancy, the healthier your startup will be. Not everyone will stay forever, or even for 4 years. Probably almost no matter what you do.