Ask HN: What's a side project you built to make money that hasn't?

A friend pointed out a bunch of the 'tell us about your successful side project' threads suffer from a survivorship bias. They're still great for inspiration, but I suspect we could learn a lot about challenges and wrong approaches from each others' failures.

So what's a side project you built hoping to generate revenue from it, that hasn't actually earned you much / any money?

Why do you think it hasn't been as successful as you thought it would be / what would you do differently if you did it again? How much time/money did you spend building it, and what kind of iterations / improvements did you make to try and salvage it?

Appreciate any and all answers!