I used to use the same identity across various websites. And kept an online, public journal linked to that same identity/username.

Things that happened, in order:

People I was friends with, in relationships with, exes would read my journal and despite I never insulted or talked bad about them, they'd somehow take umbrage.

I kept my online journal, but separated it from my other online identity, and made that journal extra anonymous.

An ex found it, and emailed me saying why was I talking smack about her/him? Thing is, I hadn't, and I asked a couple friends to read my journal post in question, and they said they had no idea what the ex was talking about.

Make even further attempts at anonymizing it, and by now I am moving the online journal around from service to service once or twice a year, making private posts older than a year, continually changing user identities, usernames, deliberately seeding misleading info (city, state, gender, and so forth).

A dude from high school somehow found it, said dude being one who'd stalked me and when I'd refused his advances he'd ripped into me, calling me a piece of shit. 15 years later, he messages me on Facebook saying he'd shared my private journal with all my high school classmates. Saying it was because he cared, and he wanted all my old high school classmates to know about the rough time I was having.

Journal came down. No publicly accessible online journal from that point on. Deleted my Facebook and Twitter accounts soon after.

Talking to people online, and despite how security conscious I am, I'd get people saying how they'd researched me, found out info about me. By this point, it was tertiary info, but still enough to concern me.

By that point and continuing to now, I deliberately change genders, never say the state or city I'm in, never use the same username twice, different passwords for everything (including any characters I create in online games or MMOs), never post photos that'd reveal personal info, and continually re-read anything I post online to see if anybody'd be able to figure out who I am, which means frequent post-editing, rewriting and deleting things.

Relatedly, Hubski is the first online forum in years I've been an active participant, and I know for a fact people'd be able to figure out who I am, find me, from my posts and comments, which means at some point I'll most likely create a new Hubski account, do away with the old, and start anew.

And for anyone who thinks they're safe online: I have never encountered someone I've chatted with online who I then wasn't able to find out real name, city, age, schools attended, job, different usernames, etc.