So, I'm currently running a 5e Curse of Strahd campaign with a party of five characters, consisting of a Tiefling bard named Tritone, a Dwarf fighter named Gunnhild, an Elf cleric named Lissius, a Half-elf sorcerer name Emilaria, and a human ranger name Romi. They've been traveling a bit in Barovia, but have spent about four sessions doing things related to or around the town of Vallaki as they're trying to keep the NPC Ireena safe from Strahd's clutches. These characters had managed to find some bones that had been stolen from the local church and killed the six vampire spawn that had been guarding them. This got the attention of Strahd, who had a letter sent to them inviting the party to dinner. The dinner itself went well, with the characters either being charmed by Strahd or not trusting him at all. He asks them to get rid of the current leader of Vallaki, a man the players had met previously and decidedly did not like. After returning to Vallaki from the dinner, the players were then invited to the house of the noblewoman Fiona Wachter, a known Strahd supporter.
The players decided to accept the invitation to her home and met the lady in the parlor, wherein they began to talk about Strahd. Since at least half of the party were favorable towards Strahd and the rest kept quiet, the lady treated them very kindly, inviting them to share an exquisite meal with her, which was more than the lord of the town had done for them. During the dinner, Lady Wachter proposed that she would be willing to take on the duty of running the town, if the party could get rid of the current leader's right-hand man, Izek. The party had already had some previous run-ins with Izek and was more than willing to deal with him. They asked if they could speak with the servants of the household to learn more about the town, and the Lady agreed, though she asked that they be cautious around her insane daughter and to not intrude on her bedroom. Thus, the players had free reign to explore the house.
Prior to this, the players had been searching for an ally indicated in a tarot card reading who was supposed to help them against Strahd. This ally was a dead noble being tended to by a grieving widow. The players had been given clues that Lady Wachter's husband had passed away several years prior, and that she still mourned him. What the players didn't know is that she has kept his body in a Gentle Repose spell up in her bedroom, and that he is their ally that they have to restore life to. However, when exploring the house, the party stopped outside of the master bedroom, realized it was the master bedroom, and then said the following phrases:
"This is the master bedroom I think…" (The ranger)
"Looks like it, but Fiona did ask us not to enter here." (The bard)
"She gave us food and treated us nicely, I say we respect her privacy and leave it alone." (The fighter)
After this discussion, the party turned around and went downstairs, leaving the door untouched and unknowingly ignoring the chance to obtain their fated ally. They did not search the house any longer and went to take out Izek in a rather gruesome manner. After the deed was done, the party witnessed a riot occurring where Lady Wachter was leading the civilians against the leader of the town, calling for him and his family to be put to death. She greeted the players with a smile and told them the town would be in good hands thanks to them, before sending them on their way.
What the players did not know due to their failure to explore the house and question her further, is that Lady Wachter is actually the leader of a demon-worshiping cult who has drawn many other noble houses under her control. The only thing stopping her from taking full control of the town was the previous leader, whom the party had deposed, leading the way for Strahd to gain a stronger foothold in the town at large.
And that is how my party doomed a town of innocents because they didn't want to intrude on someone's privacy.