Thanks for taking a look at it; sorry you found some bugs. I should point out that I only posted it publicly a week ago, and this has been my first traffic spike (thanks YCombinator!). I actually only started working on it less than three weeks ago, so I'm not surprised some boundary cases cause trouble. I've got more plans for it in the future. One thing I should note - a lot of people are working against the sample fiddles, but those are probably not where most people will be doing actual fiddling with their own queries, when they use the site for real. Different schema definitions are completely isolated (with little traffic), and so shouldn't see the sorts of problems you stumbled on.
Your creation is very cool, kudos. I don't mean to be pedantic, but I think you mean "thanks Hacker News." The people behind YCombinator are certainly responsible for this website, but they are two different things.
It's a neat concept, but it needs serious polishing before I'd consider going to this even for quick and dirty testing or demo purposes.