This is about the so-called "canon lawyer", the person with huge archives of Forgotten Realms knowledge in their head that not only knows obscure minutiae of Faerun and Abeir-Toril, they also expect any FR game to use all of this. They are notorious for makin an issue out of when something comes up in a game that contradicts something they know about the setting, especially something that most players would consider a minor detail.
I've heard a lot of theories/explanations that this was one of the big reasons for the gutting/nuking of the Realms for 4e, that these "canon lawyers" were making life miserable on DM's that couldn't keep up with the vast lore of the Forgotten Realms (that frankly, nobody except maybe somebody who was paid to do so by WotC could be expected to completely keep up with), so they jumped the timeline ahead 100+ years, killed off or drove mads loads of people and slaughtered a good number of deities all as a giant reset switch to invalidate tons of canon so DMs could run Forgotten Realms canonically without worrying about what was written before.
The thing is, I always hear these "canon lawyer" horror stories online but I never ran across them in real life. When I run Forgotten Realms the players I run with generally know the Realms from playing Baldur's Gate or following some of the novels (particularly the Elminster or Drizzt ones), or they are also a casual fan and have read some of the gaming materials, especially those about a part of the setting they prefer (one of my friends likes Netheril, for example, and knows that decently well, but he couldn't rattle off dates and obscure minutiae). They all understand that they are playing in that setting, but it isn't precisely like everything and that I am only human. I am trying to keep the game fairly close to the setting as published, certainly enough that a casual fan won't notice, but I can't promise 100% canon match, they certainly don't ask for it, and I think I wouldn't take it well if I had someone join a game I was running that really tried to throw obscure esoteric lore from some novel or some sourcebook I haven't read in my face.
I've ran into them in message boards, like when I once said something about the Forgotten Realms that is contradicted by some obscure public e-mail Ed Greenwood from years ago that was dutifully archived, and I've heard some horror stories about them here, but I do really wonder, just how common are they in typical Forgotten Realms D&D play?