Lets hear some of your more inventive ways you caught the PCs off-guard and when they were expecting ice, you gave them fire!
In one of my adventures, the PCs were told that the "Sons of Light" had vital information and played a huge part of what was going on in one of the main cities they had been in. So in their heads they were naturally thinking Pelor, or some other sort of goodly group. However I had taken a page out of the Game of Thrones mythos that I'd realized when Melissandre gave birth to the shadow-baby assassin. What is it that light births and creates? Any light source creates a shadow.
Needless to say, they went into it woefully under-prepared and non-buffed because they thought they were dealing with non-threatening entities. They thought they were going in to get easy information, and ended up in the fight of their lives. They didn't even get WTF had happened right away. At first they thought the Sons of Light had gotten abducted or taken out prior to their meeting. Took a long while for them to get it, so I was able to run with it in various scenarios and play it out really well. When they finally did understand what was going on, the looks on their faces was priceless!
I love pulling that kind of stuff, great story DMC!
I've had my players on a quest for the past 6 or 7 sessions. Two of them died in the previous "chapter" of the campaign. Their companions brought their bodies to a large city hoping to get them resurrected, but they are all broke. An order of Pelor agreed to resurrect them, but asked their companions to agree to seek a certain fruit in return. The head priest had heard a rumor of a sea captain who claimed to have found the mythical Tree of Life (or World Tree or Great Tree or Tree of the World, I've called it several names). Legend says the fruit of the tree is a great panacea that cures all ills, prolongs life, and can even bring back the dead. The priest asked them to seek this fruit and bring it back to him.
To make sure they followed through with their end of the bargain, he placed a timed curse on them. They have one year to return to him with the fruit, or proof of their failed efforts to gain the same, or die a slow, agonizing death.
So the PCs have spent session after session seeking this fruit. They made it to the island of the Tree, made common cause with some non-evil Drow, navigated the caverns below the Tree's roots, faced hags and slaadi, found the rumored entrance to the Tree's roots, navigated a labyrinth where the face four elementals and my homebrew Elemental Golem (see Everything Homebrew forum). They then found themselves in a fae revel, spoke to the Satyr King who directed them to where the fruit and final guardian are. The King also asked for a fruit to free his wife who was imprisoned within the tree ages ago by the Makers of the Tree. He also warned them to beware the guardian, saying that the guardian could only speak the truth, but was crafty and would try to trick them.
So, that's the setup. Here's the twist: turns out that the guardian is a good creature (a Coautle [sp?]) who, after much discussion, informs them that the taking the fruit away from the Tree will open a planar rift that the Tree was created to seal. He detects magic on them and tells them they are under no curse...
Then the giants attack the tree. I dragged this whole punchline out for months. Now the party is in deep dookie; in the fight with the giants, the fruit branch was broken loose (that was not planned, it happened in the fighting as a result of the players not listening ), causing fruit to leave the tree and releasing the "wife" of the Satyr King, who turns out to be an Ancient Green Dragon, and causing the tree to begin breaking apart. So, we ended our last session with the mission unfulfilled, a dragon being released upon the world, and the PCs hanging by ropes and vines from a slowly collapsing tree taller than the Empire State building.
My most recent twist happened after the PCs had been working with the mayor of a small village to undertake an investigation into the murder of a common ally from their past. In the process, they revealed the mayor's aide to be a shapechanger and the murderer, who probably also killed the original aide and took his place. They caught the shapechanger, but later discovered evidence proving that the mayor himself was of the same ilk.
No one was expecting the mayor. The discovery happened in a cave where the original bodies of both the aide and the mayor had been deposited. I heard at least one person say, "Oh s$%t!" when I described them.
The next big twist will be revealed soon. The wizard is about to discover the only way they can destroy the artifact fragments they are trying to collect is with the assistance of a sorcerer. It's a fluke of the metaphysics on my world. Anyway, sorcerers are extraordinarily rare on my world (1:500,000), and none of them advertise their abilities. Incidentally, the party just met one, defeated him, and handed him over to the local magic police to be sent away to magic prison. Next session I will reveal the means to destroy the fragment, and they'll have to find a way to break the sorcerer out of jail.
My favorite was having my PCs begin to infiltrate a church that was of some suspect. Disappearances and general unsettled feeling about the place from a few townsfolk. They chose to try and become members to infiltrate and learn more. Once I felt they were fairly convinced that this base level of the church hierarchy wasn't bad I pulled the rug out and the clerics began to attempt to charm the party. Most resisted which turned into a quite the fight seeing as how all the gear the party had was in their lockboxes and they were only in initiate robes, haha. A few good rolls and some quick thinking on attempting to disarm the clerics, made it so that no PC died.