What’s the best way to know if you have product market fit?

Paul Buchheit (Partner at Y Combinator)
Default Alive or Default Dead?

A related problem that I see a lot is premature scaling—founders take a small business that isn’t really working (bad unit economics, typically) and then scale it up because they want impressive growth numbers. This is similar to over-hiring in that it makes the business much harder to fix once it’s big, plus they are bleeding cash really fast.