The founders and executive team completely fucked it up. They kept hiring crappy/unmotivated people, prioritizing growing the team in quantity over quality, and with every added mediocre head it only amplified the morale drag when I'd be delivering huge progress after multi-day sleepless sprints while everyone else was barely getting anything done in the 9-5 office productivity theatre of standups and meetings.
I started absolutely dreading coming to the office after completing one of my kamikaze sprints. The VP of Eng had established this twisted pattern of holding a standup the moment I leisurely walked in the door at whatever hour I happened to make it in after not being seen for days, which for over a year was the weekly norm. Everyone in engineering would have little to nothing to say about their progress, and I'd drop another mother lode. It just made everyone else feel inferior, and I started to increasingly abhor their lack of progress and feel like I was the only person actually taking the opportunity seriously enough to make an effort.
It culminated in my abrupt departure three years in. And for years afterwards they failed to deliver any major new features successfully. Burned through many more millions of dollars, before eventually going bankrupt.
My mistake was not leaving earlier. That experience left such an awful taste in my mouth I've basically never made that level of sacrifice of my own life/time at another company since.
Edit:
To clarify, the hard lesson I learned at this job was that no amount of individual contribution can save a sinking ship. This particular experience was 15 years ago, and I've worked at numerous startups since, some with successful exits. I haven't seen any correlation between the amount of my contribution and the likelihood of a successful exit, absolutely none. Surely this is different for founders, but I haven't gone down that road yet. What I have seen however is some companies are far more dysfunctional than others, and the best thing you can do is learn to identify them quickly and RUN, the opportunity cost is too great.