I joined a friend's startup, it was five academics and me (I have an industry background). There were a few disagreements about the approach, I found the academics didn't have much experience about how to market a product, it led to creating a few products with insufficient feedback, and they failed when released (we couldn't find anyone to buy them, the prospective customers we had talked to and had said "yeah sure sounds like a good idea" were, unsurprisingly, reticent to buy after we'd built it).

There were also disagreements with the CEO, he was a PhD my friend had recruited, and he didn't have any experience with anything but his PhD, which led to my friend and me having to spend tons of time trying to show him how his proposed solutions to problems wouldn't work.

In the end, my friend laid me off 364 days after I was hired, one day before my cliff vested, which is very illegal under Canadian law. I got an attorney and sued them, but the attorney slowly kept ramping the cost up and up, until she told me the entire lawsuit would probably cost around $100k. I doubt their company is worth that much, so I dropped the suit, and have resorted to voodoo instead, which is much cheaper.