Not really a project, but my PhD. Did it while I was working part time and made a mess of it, finally dropped out. Now I have to explain why my CV is the way it is to every interviewer and watch them silently judge me.

If any undergrads are reading, the short advice is - don't start a PhD unless you are very good or your supervisor is very good. Look at introductory courses for PhDs to get a feel for it. Research is a whole different ball game than studying for exams etc.