What do you know about vampires? Seriously, take a minute and think about it. Off the top of my head I know that garlic repels them, they don’t cast reflections, and that stakes through the heart can kill them. That’s pretty common real-world folklore, right? But if we look at the 5e D&D version of the vampire only the bit about stakes is reflected by the monster description. This is not to knock 5e, or to somehow suggest that their vampire is “incomplete.” Their version of Edward Cullen looks like a perfectly serviceable critter, and definitely evokes that sweet vampire flavor. Moreover, I don’t think anybody wants a point by point breakdown of all the vampire lore from the Wiki page (Decapitate the body! Put the head behind the buttocks! It’s the only way to be sure!). But this kind of conflict between “common knowledge” and game-world knowledge does present a bit of a problem for GMs. If a player meets Strahd von Zarovich and tries to repel him with garlic, does it work? What about decapitation? What about sparkling in sunlight?
All of the above can, in theory, be answered with a relevant Knowledge check. A learned wizard in Golarion might know that garlic offers a degree of protection, while his counterpart in Faerûn laughs at the idea. But as a GM, do you take an additive approach, allowing details about monsters to “become correct” when a player suggests them, or do you stick to a by-the-books reading and only use the published stats?
For my money, I think that there’s good fun to be had with a “the monsters in this world are different” approach. Part of the challenge is figuring out what “rules” the monsters follows. After all, any adventurer worth his salt knows to take an “assume nothing” approach to dungeon delving. But I’ve also heard rumor of a neat variation on Knowledge skills where, if you roll high enough, you can make up a bit of lore and have it become true (e.g. Its vision is based on movement!).
Like so much in this hobby, I don’t think there is a “right” solution, but I’m always curious to hear alternatives. That brings us to the question of the day: When you find that common knowledge conflicts with game lore, how do you handle it at your table?