There's a trend in fiction as a whole regarding
subverting expectations. Playing with what audiences expect from the tropes and conventions of a particular genre, and doing something different with them. Done poorly, it can come off as feeling like a subversion of expectations for its own sake. But it can also put new spins on a concept, and keep audiences - and players - guessing.
Probably one of the oldest subversions is setting up apparent supernatural activity, only for it to all turn out to be a hoax put on by scam artists. Scooby-Doo stands as proof that you can get a decades-long franchise out of variations on this theme, and it influenced tabletop RPGs as early as The Sinister Secret of Saltmarsh, one of the biggest inspirations for the Lunar Lands as a setting. But what about a double subversion - where it seems like the monster is a fake, only for it to be very real?
Troll Sightings
Background
Valter von Sallers and Bertram Dahlman are a couple of fast-talking con artists that have been behind many a scheme all up and down these parts. Since their habits usually lead to them getting chased out of town by an angry mob once the villagers catch on to their plot, they never stay in one place for long, remaining in the area for a month or so to wring as much coin out of their marks as possible before they're forced to skip town when things get too hot. Then they'll come up with a new scheme and repeat the same thing a little bit further down the road.
Valter is a tall, skinny man with a bushy red mustache, while Bertram is shorter and stockier, with a black eye and a tooth missing; he's told many stories about how he got such injuries, which never seem to be the same twice. The two men bicker with each other, and often disagree on how to best conduct their plots, but at the end of the day they know they wouldn't be anywhere without each other, and Fate always ordains to draw the two of them back together in the end.
Valter and Bertram came up with their latest scheme after a previous misadventure left them in the possession of a supply cart heading to a barracks, providing them with weapons and armor. They have dressed themselves in mismatched brigandine and armed themselves with weapons (both have the stats of guards), even though they don't know the first thing about fighting. Now, they have arrived in town claiming to be great hunters of monsters, and warning the townspeople about a ferocious troll that was sighted not far from town!
This is, of course, nonsense - the two men want to drum up fear so that they can extort money from the village. At least, that's what they think. Coincidentally, there really is a troll nearby - and they may end up in deeper trouble!
The Encounter
This encounter is best used in a town, either one the PCs come upon on the road, or one that they are already staying at when Valter and Bertram arrive. You may wish to plant rumors of a pair of traveling con artists, monster hunters, or troll sightings before the party comes upon this encounter.
Valter and Bertram should arrive in the town square, making quite a ruckus and drawing a crowd. Bertram stands and rings a bell to get the townspeople's attention, while Valter produces a scroll and loudly reads proclamations from it. He announces that the two of them are monster hunters of great esteem who have saved many a village from dangerous beasts, and have the stories to prove it (feel free to throw in some outlandish anecdotes here). They have discovered tracks from a troll lurking outside of town - one that is surely lying in wait to eat the townspeople! The hunters are still tracking the troll with the intent of following it to its lair, but are in need of more supplies for such a task, imploring the people for donations to the cause - after all, as Valter reminds them, it is their funds that keep them safe at night when such a creature is about!
The two men proclaim themselves to be great warriors, and will demonstrate swordplay for the people, but it is obvious to anyone actually trained in the fighting arts that they don't know what they're doing and are just making things up as they go. They also claim to know a great bit about trolls, and they astonish the townspeople with their vivid descriptions of the dangers these creatures pose. You should mix in some facts that are true - such as that trolls feast on human flesh; that they can regrow chopped-off limbs; or that they turn to stone in sunlight (this is true in the Lunar Lands. I swear I have a Mandela Effect memory of trolls in 3.0e turning to stone in true Tolkienian fashion, and I've always run them accordingly) - with some that are obvious nonsense, such as that trolls are repelled by garlic or can breathe in water as though it were air. Valter and Bertram are, if nothing else, well-traveled, and they know how trolls work, but they can't resist embellishing things for gullible peasants who don't know any better if it keeps the gold coming in.
If the PCs offer to help find the troll, Valter and Bertram insist that they can handle it themselves and dare not let outsiders interfere in the hunt, lest it disturb their investigation. If they show any skepticism about their tales, the men direct them to the woods outside of town, where they have sighted troll tracks to the east. In fact, they claim, the tracks are getting closer every night, so it's important their expedition be funded soon!
Sure enough, there are giant clawed footprints in the ground to the east. These were made by Valter and Bertram in the night, using a wooden troll foot they press in the ground. Bertram keeps this in his room at the inn where he is staying, in a burlap sack under the bed. Normally, his door is locked during the day and he carries the key on his belt, but he is quite fond of wine, and it may be possible to swipe the key from him while he is drunk at the local tavern - though doing so in broad daylight may be trickier. A DC 15 Investigation check will reveal that there are leaves piled up beside every troll footprint; moving them aside reveals human footprints beside them from Valter and Bertram walking in the soft ground to place the tracks. Additionally, a DC 15 Nature check will reveal that the tracks don't seem to be spaced far enough apart to have been made by the typical gait of a troll. A Ranger with giants as a favored enemy has Advantage on this check.
However, a more thorough investigation will reveal that, to the north, there are another set of tracks similar to the first, but these have no leaves piled up beside them, and they are much more in keeping with a troll's anatomy (same checks as before apply to confirm these as real). These are from a troll that really does live close to town - and in fact, Valter and Bertram have been causing so much commotion as to disturb the troll's slumber during the day with their loud announcements and the cheering crowds they draw. If things continue, the troll will eventually see fit as to eliminate the source of the noise...
Further Developments
If the PCs expose Valter and Bertram as frauds, the people will rise up against them for taking advantage of their fear to line their purses, forcing the pair to flee town. The villagers will be grateful for the help. Although they are in no mood to give away money at this time, they will be happy to let the PCs stay at the inn for free as a token of their thanks.
However, if the troll is not dealt with, it will attack in the night after Valter and Bertram leave, barging into town and breaking into houses to devour the villagers in their beds. The PCs may be in for a rude awakening if they think things are over!
It is possible that the PCs will want to stake Valter and Bertram out in the hopes of catching them making tracks. A climactic finish to this encounter would be to have them catch the men in the act, only for the troll to attack them then and there! Valter and Bertram know they don't stand a chance against a real threat, and fall back on the PCs to defend them, begging them for mercy. Will the party be willing to stand in their defense?
If Valter and Bertram survive the encounter, you may wish for them to become recurring NPCs and have them show up in future escapades. Certainly they won't balk at any opportunity to make some money, no matter how underhanded, and if they're foiled at one scheme, they'll just go on to try another.