I enjoy a clever trap or puzzle. When it comes to favorites, two come to mind.
Once upon a time, I ran a 1E adventure for Fred and someone else who I’ve forgotten. Fred ran Blake. Someone Else ran something. Their mission was to defeat an evil overlord who oppressed the people of his domain. Justifiable fear of assassination motivated said evil overlord to lair in the center of a maze rumored to be impenetrable. From within the maze, evil overlord sent forth his monstrous minions to do his wicked bidding.
Blake and Something entered the maze. They could not help but notice the strange runes over the archways leading from one maze chamber to the next. The heroes decided that the runes must be the key to navigating the maze. So, while fighting off one ambush after another, and never discovering from whence the ambushers came, Blake and Something made meticulous notes about the runes and applied their code-breaking skills.
There was one huge problem with this approach. The runes were a red herring. They had no intrinsic meaning related to the correct path through the maze because there was no correct path through the maze. Indeed, there was no maze at all. The entire labyrinth was a complex and powerful permanent illusion situated in a large cavern beneath the evil overlord’s castle. The evil overlord and all his monstrous minions were immune to the illusion. Blake and Something weren’t.
After a couple of hours, Blake said something to the effect of, “This doesn’t make any sense. There are no secret doors anywhere. The runes aren’t helping. I can’t believe anyone could get through this maze.”
I said something to the effect, “Save versus spells for Blake.”
A die clattered. “I made it.”
“The maze isn’t real. It’s an illusion.”
Much cussing followed, and then Blake and Something pierced the veil and brought the fight directly to the evil overlord.
In another adventure for 2E that I wrote as a sidetrek for Return to the Tomb of Horrors, the PCs had to retrieve a magical key. The key was at the bottom of a dungeon, of course. The heroes fought their way through the hordes of undead to the key’s chamber, which was described something to this effect:
“The chamber is a large rectangle, at least 60 feet long and 30 feet wide. Two rows of ornate stone columns to the left and right support the arched ceiling. The far wall is pitch black. About 15 feet in front of this black wall is a pedestal. Floating over the pedestal in a shimmering light is the key. A tendril of light extends from the black wall to the shimmering sphere that holds the key.”
Investigation revealed that the black wall was transparent. Behind it in inky darkness swam undead things, including an enormous snapping turtle skeleton. The shimmering sphere around the key seemed impervious, as did the tendril of light.
“Let’s see if dispel magic will work,” someone said.
They did, and it did. What the heroes didn’t know is that the shimmering sphere of light was not a product of the transparent black wall, but vice versa. Once the shimmering sphere of light was dispelled, the heroes could get the key, but only after the transparent black wall vanished and hundreds of thousands of gallons of cold water full of undead monsters rushed into the chamber. Better still, the columns that supported the ceiling didn’t really support the ceiling. They weren’t solidly anchored to either floor or ceiling, and the surge of water toppled them to inflict crushing damage on whomever they fell.
Ah, good times.