As a general rule, I never adjust a combat encounter’s stats on the fly, even if what was supposed to be a tough encounter becomes a cake walk. If that does happen, I try to find a way to complicate the scene by calling in reinforcements or using a fireball spell to set the room on fire and ramp up the tension. But almost never by adjusting the details already prepped and recorded.
Back in my 3e home brew campaign, I wanted to pit the PCs against a massive horde of flesh-eating zombies. The idea was that they had to survive the night as these zombies would turn to dust at dawn. It was designed to force the PCs out of their standard fight-in-the-open approach and add some mobility to the encounter as they had to move from one secure shelter to another to outlast the night.
Uh, yeah, that’s not what happened. The sorcerer had just learned a polymorph spell that would allow him and two others to turn into a creature they encounter before. So in their waning hours before the horde attacked, they chose a large insectoid creature called a “roo-ken-eer” from the previous adventure and decided to hold out for as long as possible in (sigh) a big fight out in the open. Well, that choice of monster just happened to provide the PCs will the PERFECT immunities and damage reduction that allowed them to survive hundreds of zombies attacking as a horde AND it also gave them the ideal attack that would allow them to sweep through the horde. Once I realized they had (by sheer luck) picked the perfect defence and therefore nerfed all my plans… I gave it to them. They planned, took what they knew about the situation, and made all the right choices to win the battle. In that moment, they deserved the complete victory.
…in the next adventure, they had to go up against a pit fiend. They were something like a CR10 party or something. I needed to take something from them and wanted to make sure it would get done. So I took the main boss for the whole campaign and brought him in for an early appearance. No deaths, no injuries, just some dislocated jaws as they saw a pit fiend take the sorcerer’s younger brother into a portal to Hell. They were not wearing brown pants that day, let me tell ya.
So maybe I do adjust things… but later. Once they’ve proven themselves capable of kicking ass in a tough encounter before the final act of the game, standard pacing and plot devices tell us to bring them down a notch as their nemesis tries to quell this growing threat. You blow up the Death Star, the Empire’s gonna crush you with giant metal camels.
And this DM don’t fuck around when you wipe out his zombie horde.