I've been through a few startups, but one experience really has stuck with me. I was an early employee in a turnkey software startup built on ideas from a moderately noted researcher in the field the software was designed to sell to. I had known the researcher off and on over the years and he had been brought on as the CTO with the investors bringing in an outside person they knew as the CEO.
They were still in "stealth mode" at this time, taking some earlier work the researcher had spent time on and hardening it for general commercial sale. This was the first red flag I should have seen, instead of taking the time to reboot the product and start it over for a modern market, the development team was spending a year doing bug fixes and cleaning up broken research code.
Not long after starting I went with the CTO to a trade show for market research and meet and greets with some industry partners. He booked us a lavish 4-star hotel and during a week-long trip maybe spent 2 hours at the show. He couldn't get out of there fast enough and instead wanted to rent a conference room outside the city and spend the week brainstorming product ideas. Remember, he had directed the dev team to not develop a new product but to merely fix and update his old code. He then described some new ideas that I recognized as being more or less exactly like a competing product. I asked him to describe how we would differentiate it from that product. Instead he claimed it was a totally different idea. Red flags #2, #3 and 4
And it went on a for a while, intense conflicts between the CTO and the dev manager put development work into a holding pattern, lack of a good sales strategy, and so on.
Eventually there was complete turnover at the C-level and the investors brought in a new team, they asked me to stay on while they rebooted the company so I did, and we did better, almost turned it around. But by that time we were stuck with a huge legacy codebase dwindling investment dollars and a host of other problems that we could never seem to shake.
Despite being a bad experience, I learned more from that failed company than I've learned in the other 15 years of work. Observing so many failure modes happening all at once, and seeing how a pro-team might go about a turn-around has taught me an entire business school's worth of street smarts.
I'd almost recommend riding a company down like that to people serious about startups so they can learn the same things, but to be honest, I wouldn't wish it on my worst enemy.