So many games have big reveals, where the sheet is lifted partway through and everyone is (hopefully) surprised by the twist, which shapes the very course of the game.

I'm not looking for those!

I'm interested in the small reveals which have happened in your games - anything from referencing another character which someone in the game played in another game with you, to someone turning out to be someone or something different in a manner which was surprising and interesting but ultimately not game-changing.

I'll open with when the party combined a bag of holding with a portable hole and went on a short adventure to the Astral Sea. In my world, all bags of holding dump into the same place, which has formed an island in the astral sea. There, the party members found a chart which mentioned a lot of places they didn't recognise, including one called Benfrost, which is one of my players own homebrew worlds in which she runs her own games. Her face when I casually dropped it in as something they find but don't understand was amazing!

I don't know of this counts, but I know someone who always plays their character with the same name. Its part of their backstory that all of their characters are related to each other across the multiverse.

I'm a forever DM, and I always like to sprinkle little easter egg tidbits for my friends into new campaigns. I have exactly one returning character, a gnome, who was a terrible and evil little shit during the first campaign we ever played. Over other campaigns he's been different, sure, but his name and his gnominess remain. 

I'm currently running a multiverse spanning adventure, so this campaign is ripe with tiny reveals and surprises. Tried PCs from previous games have been introduced, they traveled to an old homebrew world, and even took a class in Sigil that was being taught by a few members of Critical Role's Vox Machina! That was a fun reveal. (It was Grog teaching, at the behest of Pike who wanted to show him that he can do 'smart people' things, too).