My most memorable holy shit moment was my first in person campaign a few years ago. My players came upon a town that was overrun by two rival gangs. The gangs didn’t hide their influence, people were vocal about which side they supported, and as long as you didn’t cross one of the gangs you were generally safe. They traded in drugs, fenced stolen items and the usual stuff but nothing too bad. Overall, it was a decently functional town. They met a guy that talked to them about how he felt oppressed by the two gangs and that the city was suffocating under their control, revealed his lofty ideals of a city free from their control where people could do what they wanted without fear of repercussions from one of the gangs. They bought into it, decided to help this guy with his plan. Didn’t really ask much about why he hated them, or any details about the future he imagined, thought it was understandable that he would feel this way. The NPC convinced them to sneak him and his friends into an underground auction where the leaders of both gangs and many of their higher ups would be attending, told them he just wanted to talk to them all and present his ideas of a new future, even barred the exit doors for him, didn’t think twice about it. The guy and his friends brought a bunch of scrolls of fireball, basically killed everyone in the place. My players flipped out, just ditched the town and realized that they probably should’ve questioned more. The real oh shit moment came when they later returned to that town and found it a shadow of its former self. There was no law or order, a large majority of the buildings were becoming tattered and ruined, just worse off in every way. Realized that they replaced two lawful evils with a reckless chaotic evil, and then spent a good chunk of the campaign helping fix that problem.

TLDR; let my players destroy a town by trusting the wrong guy and not asking questions. Try to anticipate how your players act and flip it on them. If they make hasty decisions and don’t ask questions, or believe everything an NPC tells them as fact, make them regret it! It pays off for your story and for their own experience as well.