I have one I've told publicly before. Once more, with feeling!
I was a moderator at a special interest chat which did not welcome anyone under 21. (Its previous incarnation was pretty much ruined by teenagers, I understand.) HarmlessHarry* kept overriding the auto-censors to include fairly raw language, which the chat did not allow. (He'd write things like sh!t or fvck.) As moderator on duty, I warned him several times. He stopped doing it--when I was moderating.
But one day I'd dropped down from moderator status, leaving the chat without a moderator temporarily while I talked privately with a friend. HarmlessHarry didn't notice me well down the alphabetized member list. He started up with the not-allowed language immediately. I said, "Knock it off, Harry" and his response was, "Holy fvck, there's a mod!" So I banned him. Enough, already.
HarmlessHarry did not take this well. He contacted the chat's owner via email, and she backed me up.
The next part is mostly hearsay, and I didn't learn about it until well after it happened. HarmlessHarry emailed and instant messaged his friends from the chat. He learned my real first name (which is fairly unusual) from one, the general area I lived in from another, the nearest city to me from someone else, the name of the grocery store I shopped at from another person, obtained a photo of me (which I had posted to a private group, not accessible to anyone but other mods) from a mutual friend, and so on. He also gathered information about the chat's owner, who is far more findable because she's an on-air reporter for a TV network in a major market.
A short time later, HarmlessHarry's wife emailed everyone in his address book. Harry had stopped taking his medications weeks ago, she'd just learned, including diabetes medication and antidepressants. He'd left home without explanation, and his handgun was not in its place. She feared he meant to harm someone. Did anyone know where he was? Had they heard from him? An acquaintance from chat told me about the email. I was disturbed, but not enough to call the police. I didn't know at the time how much information he'd gathered.
HarmlessHarry shot himself in a motel room about 80 miles from where I live. As far as I know, he never learned my last name.
And that is why I never, ever use my full real name online. Eventually, I'll piss somebody off, and you never know who's off their medication and genuinely deranged and dangerous.
* HarmlessHarry is similar to his real user name, but I've changed it