If one has a "red flag" in their history (in terms of non-academic issues), is it best to disclose it to a potential supervisor? This can apply to any type of situation (a conflict with a faculty member, another student, a mental health issue that affected one's behavior etc.) Obviously, one wouldn't want to disclose something minor when the supervisor/university probably wouldn't hear about it anyway; I'm referring to situations that reach administration and that might be mentioned in a "letter of recommendation" (or when there might not be any letters from one's dept. etc.)?
I would treat this like dating. You'll need to be honest and disclose red flags before you get engaged/married. But it doesn't need to come out on the first date. The reason is that so many other things might happen that would make disclosing the red flag totally unnecessary.
For example, you write a professor expressing interest in working with him/her, and he/she writes back "sorry I'm going on sabbatical next year and not taking new students", or "sorry my lab is full right now I don't have room for new students". In those cases, disclosing a red flag is pointless. He/she isn't going to work with you anyway, so why bring it up?
Or maybe, he/she writes back "yes I would love to work with you, I have an immediate opening for <extremely boring project>". In that case, you don't want to work on that boring project, so why bring anything else up?
Now if you've exchanged a few e-mails, or a phone call, and it looks like there is some mutual interest, you are interested in his/her projects, and he/she is interested in you, but before you have formally accepted the position, that's when you should probably say, "before we go any further, I wanted to be honest with you and let you know about <potential red flag>."