The big one for me was realizing that my boss had lied about knowing how to code. Other red flags include code and documentation quality, how the business sells itself to potential customers (does it greatly oversell it’s abilities), the expiration date of the coffee/tea in the break room (I got food poisoning when I drank the startup’s three year expired tea - the founders were still wondering why they felt like crap all the time when I left), etc.

It’s really a bunch of little things that, when seen as off all at once, will trip your spidey sense to jump ship.

Mostly it's business growth stalling with no clear plan to fix it.

If you don't have visibility into that, there are other warnings signs, like various spending cuts. No straight answers to questions about that stuff.

Not providing visibility into the business is also a warning sign, in my opinion.

I've also left companies when they started making pretty weird business bets that I dind't think would pan out.