There’s a meme going around Facebook today: Rejected Princesses. I was reading along, and suddenly I went, “Hey! That’s where Nzinga, Seated got her username!” 
(Okay, maybe everybody else already knew, but I’d never heard of Nzinga before today.)
I was also reading a Harry Dresden book recently where one of the villains had the nickname “Shagnasty,” but I think our Doper of the same name predates the book.
Anybody else ever spot one of these and have a little “oh!” moment?
How pathetic is this?–I’ve only been back on The Board for a few months, and saw the handle, “Dr. Girlfriend.” No idea what that meant, never heard of “The Venture Bros.” Jonny Quest thread mentioned TVBs, I give it a whirl, it’s hysterical, one of the characters is DR. GIRLFRIEND!! I hope Straight Dope’s DG’s voice isn’t quite that low and raspy.
I always thought Saltire referenced “salty satire”, then I saw Black Watch (a play about a Scottish military team in the middle east) and there was a bit at the end involving the saltire of the Scottish flag. Turns out “saltire” is not a made-up word, but is the criss-cross part of the Scottish flag. I now think of Saltire as Scottish.
There are quite a few users here who have taken their user names from some existing fictional (or sometimes non-fictional) source, many of which I had never heard of before. Every so often I stumble upon seeing one of those names in another context (typically, the original usage or some reference thereto). Just to name one example among many, we have a user here formerly known as Inigo Montoya that I had never heard of outside this board until by happenstance I did.
I can’t remember which posters but yes, on more than one occasion.
All the time! I can’t for the life of me remember any right now.
I learn the origin of at least one new name every year when I go to a 36-hour sci-fi film marathon.
Okay, I just spotted a case within the last few minutes.
We have a Doper here who calls himself sleestak, n’est-ce pas? Never heard of that otherwise until today.
Until this thread by Czarcasm, with this post by Typo Negative.
Doing a bit of research, one discovers: Sleestak.
Didn’t we used to have a poster named ratatosK or ratatoskK?
I was just reading something about Norse mythology, and I learned that Ratatosk is the squirrel who runs up and down Yggdrasil, the world tree. I thought, “Heeyyyyyy, I’ve seen that word before!”
C K Dexter Haven. And I had seen the Philadelphia Story years ago but just didn’t retain it. A recent reviewing had me forehead-smacking.
I was recently reading a book about Bletchley Park, “Secrets of Station X,” when I found out what ** Captain Ridley 's Shooting Party ** was.
Never read much Alan Dean Foster. So it was a while before I found out where Clothahump got his username.
You never watched Land of the Lost? You must not have been a kid in the 70s. 
Marshall, Will and Holly
On a routine expedition…
I haven’t read any posts by him lately, but we also have had a Roger Thornhill post here.* Apparently Cary Grant is a popular guy to Dopers.**
I’ve also seen several Doper handles pop out at me from the page since I started reading Pratchett.
Rincewind, Twoflower, Bill Door, Ponder Stibbons, and SusanStoHelit to name a few.
*banned about eight years ago, apparently
** Roger O Thornhill was the protagonist in Hitchcock’s North By Northwest
I had seen the movie, This Island Earth well before joining the boards, but never noticed Cal Meachem was the protagonist’s name until I was looking up something else in IMDb.
I didn’t know what Colibri meant, or even that it meant anything in particular at all, until it was mentioned in some thread rather recently.
Some I knew but was never sure of: was TVeblen a reference to the economic philosopher who coined the term “conspicuous consumption?” Guizot for the French statesman/historian who described the reign of Napoleon III as a pack of bastards?
OurLordPeace, however, was just a string of phonemes until a neighbor explained who Jesus was to me. Yes, sometimes life really is like a Chick tract.
It’s not Scottish. It’s a heraldic symbol describing that form of diagonal cross like the St. Andrew’s Cross on the flag. But it’s on others like Jamaica and Florida.
I think I remember in one of the “How did you come up with your username” threads that it was a play on the name of the band Our Lady Peace.
And ** Mrs Cake ** !
Stranger on a Train. The “aha” moment was when I read a review of the film in a book by Roger Ebert; later, I rented it through Netflix.