One comparatively common rumor I remember back from WoW classic had to do with the so-called "Cutthroat Alley", a section within the Dwarven District in Stormwind that never saw had no NPCs or much of anything else safe for a two-story building in it.
Cf. wow.gamepedia.com/Cut-Throat_Alley
Now, the rumor went that, at certain dates, a NPC with the blonde human child model and the fitting name "Young Girl" would spawn and remain in the second story of the aforementioned building for a few minutes. Every player used to have his or her own theory just when the girl would spawn, with a popular example being that the numerical values of the date and the server time had to add up and produce one specific number, causing the girl to spawn.
The girl herself resembled most other child NPCs - same model, could neither be attacked by Alliance nor Horde players, had no voicefile and no text dialogue. However, she markedly differed from all other NPCs in the game in that she could be targeted by a little fun item - a kind of ball which players could throw to other players, removing them from their inventory and adding it to that of the enemy (sorry for no longer recalling the name, it's been a while). At any rate, this should not normally have been possible because (regular) NPCs simply were not an applicable target for the "active function" of this item and all you'd get was an error message.
Throwing one of these balls at the young girl initially did not have ostensible effect - the NPC would not even turn towards you. But it actually would cause a different NPC - a gnome merchant named Figerius Fizzlewicks - to spawn in what were thought to be one of three locations - the catacombs under the slaughtered lamb, those under the cathedral, and in an otherwise empty shop in the park (again, mind you that this was back in classic!)
Now, this Fizzlewicks fellow you could trade with; the odd thing being that his stock - judging by those who claim to have interacted with him - was more or less random. You'd have anything from consumables for players just out of the starting areas to rare weapons and armor that normally would only show up as world drops and were bound-on-pickup to highly sought-after stuff from the old Zandalari faction when Zul'Gurub still was the hot shit.
Some people on the realm forum claimed that Mr Fizzlewicks was a pet project by one of the game's coders, and that his stock actually consisted of copies made from items in the inventories of characters that were deleted after their players did the same to their respective accounts, but whether or not this is the case is largely negligible. What -was- interesting was a white quest object named "Prime Physics", which supposedly used the same inventory icon as the famed Gnomish Death Ray. And sold for the hefty sum of 10120 gold (which would have made Scrooge McDuck cream himself back when people endlessly grinded for their 800 gold epic mount).
Due to the short timespawn for which both the Young Girl and Figerius Fizzlewicks would be around before despawning, several organized attempts to raise enough gold and buy the "Prime Physics" failed short (and one guildmate of mine claimed there was at least one big scam resulting in many salty tears). I don't actually know whether anyone ever managed to get a hand on that item, but of course you had rumors built on rumors - that it was part of an ARG, that Blizzard would offer a job or lifelong free account subscription to the guy who used it, or that it produced a quest whose description actually was the address of a darknet side with godknowswhat on it.
As the "young girl" hasn't been seen since the launch of Burning Crusade (and trust me, there were enough autists camping at that spot), the general consensus is that she and the gnome were removed and likely were just a joke on the part of the devs or a beta leftover. Shit man, I don't know.
