I love moments like that. Give us some details!
Okay… wall of text incoming.
One of the players' characters back when we first started in LMoP was Kragorn, a dwarf druid with the Haunted One background - something traumatic had happened to him, and he couldn't remember anything from before he was found in the woods and raised by a group of elves.
From the start, I knew a few things: one, we were eventually going to play Curse of Strahd; two, there are no dwarves in Curse of Strahd; but three, I was going to make some pretty major changes to parts of the plot, because a couple of the party were sort-of aware of bits and pieces.
So I needed to get dwarves into Curse of Strahd, but also have something amnesia-inducingly traffic occur to (at least some of) them. If you know CoS, you'll know the ruins of Berez, a village in the south of Barovia that (in the book) was destroyed by Strahd after they hid one of the reincarnations of Tatyana from him, and is now inhabited by the crone Baba Lysaga.
So I rewrote almost all of the narrative stuff for Berez. Dwarves are common enough across the D&D multiverse, and Barovia already has elves, so I decided that Berez was a dwarven village. I kept Baba Lysaga, but changed her scarecrows into a newer monster called stone cursed, with the explanation that they were what was left of the original dwarf (or, in Barovian parlance, Berezi) inhabitants, petrified then animated as guards by the hag. Then, I added a new location into the notes: The Bailstone House.
In session 17 - some time around January 2020 - the party met a group of Vistani on the road - two NPCs from Curse of Strahd and an extra one I made up, who presented themselves as travelling merchants. They sold the party some magical items, and one of them offhandedly referred to Kragorn as a Berezi. Nobody passed any remark. The term also came up once or twice after they reached Barovia.
When the party finally arrived in Berez last session, the now-dwarven Burgomaster's ghost (who already only really exists in the adventure as an exposition machine) was able to point them towards the Bailstone House, where they discovered and were attacked by a great horror monster: The Lonely, a harpoon-armed creature of pure misery, which only wants a hug, but deals massive psychic damage if it succeeds in getting one.
They killed the thing, and while exploring the house afterwards, discovered a journal, which (short version) explained how a woman, desperate to save her son from life in Barovia, visited the Amber Temple in search of a ritual to get him out. She found one, and it worked - but it consumed all the life in the village to do it.
I kept Kragorn's name out of it until the last sentence.
I gave the journal to them, then muted myself while I waited for the reaction.
…what!? What the FUCK!?