Yup.
A woman I got to know from a blogging site (and met in real life) was snarking on the getoffmyinternets forums about a third blogger, under a totally different nick. I recognised some of the life experiences she referenced in her snark, but haven’t told her I “saw” her there.
Yeah, I use the same username on most of the boards I frequent. And I’ve recognized at least two people from here on Reddit - just by their usernames, though. I wouldn’t have put two-and-two together otherwise, there are too many users there to identify any one person as the same from another board.
The only reason I can think of to use different usernames would be if someone was prancing around the internet pretending to be something they’re not … although no specific examples of this come immediately to mind.
When I was on wedding message board, a poster gave a highly specific description of a gym she used to attend; I had a strong suspicion the poster was the former girlfriend of one of my husband’s coworkers (someone I used to socialize with, but was never close enough to maintain longterm ties with, and most importantly, we went to the same somewhat unique gym).
I private messaged her and I was right! Super random!
JFYI there are other reasons, sound ones, for not leaving a trail all over the internet of your personal life. As well as avoiding trolls and flamers following you from board to board, trying to pick a fight.
Some of my reasons:
Habit. I started using a different name on nearly every message board without really thinking about it, and “make up a new name” is just part of the habitual signing up process for me.
Theme. On specialized message boards people often like to use a name that fits the theme.
Other. My City of Heroes board name “Obscure Blade” was the same as my in-game Global Name. It’s also one of the few I’ve re-used, since CoH was shut down.
My email, X-Box handle, chat and Facebook, Reddit and Wikipedia, interent forums and comments on blogs and publications used to all be under the same.
Boy was that a mistake.
There was a recent member here who sounded vaguely familiar. Can’t know for certain, but he did get banned so maybe I was right.
I am nearly sure one of my facebook friends posts here. But I’ve never confirmed it because I don’t like having my online circles get co-mingled, so I assume she might not as well.
Why is it a bad idea to keep the same name? I understand the need for a different name if going to a health board and posting personal details, but are there other reasons?
Edit to add: Yes, people from a different board have sent me private messages of greetings.
Let’s put it this way. Is there an advantage to the poster to be connected with another board?
Case in point. Years ago, I was accused of a crime (and the crime was in the eye of the beholder only; it was a personal vendetta), and the Internet-savvy accuser did his damnest to find dirt on me. The only thing he found were two posts I had unwittingly done under the only name he knew of mine. He used both as “evidence” in his accusation, even though he had to twist both beyond recognition.
Thank Og he wasn’t able to connect any of my other posting names, or it would have been a nightmare for me to defend against.
I learned my lesson the hard way. Here’s hoping you don’t have to.
Once on a musicians board, I came across a rant regarding YouTube, the details of which were identical to a then current Pit thread. Different user names, but the same story and same tone - I figured it had to be the same person.
A word to the wise is seldom sufficient.
One word. Stalkers.
Not a cut & paste job? It’s been known to post identical stuff on more than one board and it might not be the same party.
I have a growing suspicion that a somewhat frequent poster on this board, who I won’t name, is my brother. Same writing style, same condescending attitude, even similar turns of phrase. I almost contacted him privately once but thought better of it because if it is him then I would never post here again, which would suck.
OMG. Sis, is that you?
I know one. He doesn’t know me, though, because we post on different topics and I have a completely different username on the other board.
I’ve always had a sneaky suspicion that Bad Astronomer moonlights over on Salon.com 
But seriously…
There used to be a handful of Dope regulars that were also regulars on TheOneRing.com, but it’s been many years since I frequented that site. Some I recognized by posting voice, others by similar usernames.
Wow… umkay… there’s a blast from the past. Then there was NotOkay or something… I wanted to make a new acct at that time and be ParKay.
In a slight twist to the OP’s premise, I’ve been recognized here by a few people that encountered me under another name on another board.