I hope you will forgive me if I get a bit wonky.
Thanks in advance.
Many years ago I spent an entire Summer studying demographics (some details here as to why a 10 year old would do that) and concluded that I would reject what was being written about by lepidopterists, science fiction writers, and others about overpopulation and, rather, agree with actual demographers, all of whom insisted overpopulation wasn't a problem then and wouldn't be for a long, long time. The doomsayers of overpopulation stated 1970 was far too late to prevent hundreds of millions of people from starving to death in the 1970's and that nothing could prevent famine from wiping out England by 1980.
Turns out the demographers were correct and the popular voices were wrong.
Now several major nations are grappling with rapid underpopulation and the contraction of world population should begin within my own lifetime.
During Desert Shield I encountered a group of journalists, the leader of whom was very excited. Why? He had a big scoop - he said he had caught the army lying. You see, the army had said the PATRIOT missile system was operating at more than 90% success but he had proof - proof! - that less than 1 in 4 launched missiles was even reaching the target; the rest were blown up in mid air remotely!
"Of course, that is how the system works, but it doesn't change the success rate."
You see, when a potential target is spotted 1/2 of all available launchers fire an intercept missile. Why? Safety! There isn't enough time to launch one at a time, so you launch multiples in case the first or even more miss, and then remotely destroy any that aren't needed.
I spent the next two hours fruitlessly trying to explain the critical difference between 'individual element accuracy' and 'system success rate'. He ignored me, broke his "scandal", and made headlines.
To this day I know people who know the army lied about the success of the Patriots because of that journalist.
This phenomena where someone who is incompetent at something but believes they are competent at it, even has a name - the Dunning-Kruger Effect. This state, in very simplified terms, that people who are unskilled in a certain task tend to honestly believe they are very skillful, even masters, of the task while people who are very skillful at the same task tend to rate themselves as mediocre.
"Wait a minute, Rick," you say, "I've heard of the Dunning-Kruger Effect, sure. But I know/googled/looked up/etc. overpopulation and the Patriot missile system and, well, they are terrible examples because overpopulation is a huge issue/the Patriots don't work!"
Actually, we're just getting to the point of this post, so thanks for chiming in.
Because this post is about misinformation in your campaign.
In my campaigns there are things everyone knows to be true that aren't and things everyone know to be false that aren't. And you might need to consult a sage to find this out.
Here is an example from Seaward:
Maury looked grim, "My divinations and research have explained the strange things that Sessy saw and heard; the new master of thieves is a rakshasa."
"A rakshasa!", exclaimed Sessy, "this is foul news indeed."
"What is a rock-shasta and if it bleeds, why do I care?" asked Eirik.
"Rakshasa," corrected Maury, "a magical creature from a far land. Virtually immune to magic and proof against all but the mightiest enchanted weapons it cloaks itself in a nigh-perfect illusion of being a person or creature you trust and then destroys you when you are unwary. They are cunning and powerful."
"But not invincible," added Brother Reynaud, "I recall hearing from another cleric that the merest scratch from a crossbow bolt that has been Blessed can slay them."
"I had heard this rumor, as well, " agreed Maury, "and my research in the Imperial Library confirms it. We shall confront the evil with Eirik and his henchmen armed alike with a score of Blessed bolts!"
[2 weeks later]
The series of ambushes and traps from the thieves had been bad enough, but the two doppleganger servants of the rakshasa had been even worse. The adventurers were all bleeding from various injuries and Sessy was on her way back to the surface with half the surviving henchmen guiding her; the poison-induced blindness should wear off in a few days.
With surprising ease they made it to the Master's Room. The interior was well lit and empty except for a figure lounging on the gem-encrusted throne in in the far wall. The figure looked like a tall, powerfully-built man with the head of a tiger. The rakshasa put down his hookah pipe and smoothed his silk robe as he stood, revealing that his thumbs were on the 'wrong' sides and that his fingers curled backward, not inward.
Brother Reynaud called out, "Prepare for your death, foul one! Your doom is here!"
It took a moment for the adventurers to realize the rumbling growl was a chuckle.
"Fools," growled the creature, "my minions and traps have done naught but prove you cannot harm me."
The rakshasa strode toward them, it whiskers twitching.
"Fire, men, fire!" called Eirik as his 3 surviving henchman joined him in launching crossbow bolts. Two struck true with Eirik's own quivering in the beast's throat. Soundlessly the rakshasa slipped to the ground.
Eirik leapt forward with a shout of triumph. But as he prepared to collect the thing's head it leapt up, snarling. After a swift exchange of blows Eirik drew back, bleeding from half a dozen new wounds. The rakshasa paused to pluck the bolts from its hide, the wounds sealing up instantly. He briefly sniffed one of the bolts before tossing it aside.
"Run," hissed Eirik, "run for the surface. If I live I will rejoin you."
As players in my Seaward game now know, very well, rakshasa are not harmed by crossbow bolts that have been Blessed. Crossbow bolts that have been Consecrated, however, are instant death to the horrible creatures.
Yes, my players were horrified. Yes, at least one was indignant ('but the Monster Manual says!'). But why not? After all, plenty if people think that 15th century Europeans thought the world was flat. They didn't. Indeed, the main opposition to Columbus was because the majority of scholars agreed on the circumference of the earth and expected him to run out of supplies before he reached land. Columbus was wrong about the Earth's circumference (the scholars were really close, actually), there was just a landmass or two that were not as well known in the way. But not only is this belief ['15th Century Europeans thought the world was flat'] common it can be found in school textbooks and even books in college reference libraries.
So add some facts to your campaign and make some of them fun, interesting, and true ['adding a drop of red dragon blood to the components of a Fireball makes it hotter' and give a +1 per die if they do this; 'the ichor of a slithering tracker makes you immune to a gelatinous cube's paralysis for an hour', etc.] make some of them them fun and false ['adding a drop of blue dragon blood to the components of a Lightning Bolt makes it more powerful' when it doesn't; 'if you tie the death shroud of a murderer across your face as a mask ghast stink doesn't affect you' but this doesn't work, etc.]. Also have things that 'everyone knows' be false [like the rakshasa] and that everyone disbelieves be true [for example, only superstitious peasants nail brass keys to the lintel of their door frames with a single iron nail - educated and sophisticated people sneer at this superstition.
But what if it does prevent intelligent undead from entering the home? Maybe it only works for actual family homes with a relatively small total square footage, or an average value or less, or some other limitation that means it doesn't work on castles, or wizard's towers, or churches, or the town homes of rich merchants but it does work. That isn't going to break anything, skew the campaign, or help players with their lordly manors, but it might be an interesting plot point and, is handled right, can really mess with the players.
You can do this with NPCs, too. Everyone, but everyone, knows that Kregar the Shining is the best swordsman in the West. He has been challenged on a number of occasions by renowned swordsmen but has always prevailed. People come from distant lands to train under him but he selects just one man every two or three years. These men go on to state with great pride they were trained by Kregar and laud his mastery far and wide.
He is beloved in the city, too. He tips well, is generous to friends, gives freely to the poor, and has been known to help out young, down on their luck adventurers from time to time with cash and introductions. He is charming, friendly, and soft-spoken.
In reality he is a 5th level fighter with a 16 Dexterity who is specialized in the broad sword and has a magical sword that means he always strikes first (although he has no idea it is magical). So he is pretty good, but not that good. However, he is truly convinced in his heart that he is the best swordsman in the world.
In addition to the personality traits listed above he is also prone to 'humble-bragging' such as;
'I hope you do not mind having dinner with me. If I am busy when the general comes perhaps he will stop pestering me to train the king's guard'
'I grow so weary of famous swordsmen coming just to challenge me to a duel. It disrupts my training so much.'
'Perhaps you will accept this as a gift? it was sent by a master swordsmith from a far land, but the humble blade I found years ago has been more than enough for me to win every duel.'
This can go the other way, too. Just think of Aragorn in LotR - heir to the kingship of all Men of the West and dedicated to protecting the frontier from evil the locals called him 'Strider' and though he was a disreputable sort, going so far as to warn travelers not to associate with him.
And remember, none of this has to be purposeful or malicious. These could all be no more than honest mistakes!
But why do we do this? A few reasons.
First, it cuts down on meta-gaming. Players that have memorized the books will have less of an 'advantage' in these situations.
Second, it reflects Real Life at least a little. We all are subject to this sort of mistaken confidence and false knowledge, so why not your characters, too?
Third, it makes your world unique. This is a simple way to differentiate your campaign world from anything else.
Fourth, it makes in-character research and divinations more valuable. Access to good libraries, the casting of expensive divinations, and travel to distant sages suddenly are all worth it.
And last, it adds to the sense of wonder that makes the game more fun.