But there’s a second caveat here: YC invests in people, not in ideas. From pg himself: https://www.ycombinator.com/blog/the-reddits
> Their idea was bad though. And since we thought then that we were funding ideas rather than founders, we rejected them. But we felt bad about it. Jessica was sad that we'd rejected the muffins. And it seemed wrong to me to turn down the people we'd been inspired to start YC to fund.
So it might be worth applying with a particularly bad idea. Or at least it used to be; I have no idea whether YC ever encourages strong founders with a bad idea to re-apply with a better one, or if you get any feedback from the partners that they almost accepted you but rejected because of the idea. (One of my neat memories as a 19yo is getting a thoughtful email from pg saying we almost made it to the interview with an idea that was essentially “blackboard sucks; make better school software," but the biggest factors against us were my age and the fact that selling to schools was about as hard as selling to governments back in 2008.)