Two spring to mind:
One, is in a campaign I wasn't actually participating in, but know several players who were. One player, who was only in the game for a single session, had obtained a polymorph potion of some type. he drank it unseen, approached the rest of the party in the form of a duck-headed two-legged freak of some kind and said...
"Awk! Awk! The eyes of the Great One are upon you!"
...and walked away. None of the other players had a clue what the hell was going on. As the campaign proceeded the other players convinced themselves that EVERYTHING that was happening was in some way connected to "the Great One". The player responsible never knew because he never played again and the DM didn't let on for several years real-time and then only to those players who started to grasp the reality.
- - -
The second was one of the most fun nights I've ever had playing D&D. It was in a VERY long-running game. Our PC party called themselves The Avengers. They had adventured long enough to become rulers of nations, captains of armies, movers and shakers of world events. The roster of PC's was probably 20 or more. There were games where we had 10 and 12 players at a time and it was very common for players to run two characters at once. The level of these characters ranged from low single digits to high teens and eventually one 20th+ level.
There came to be, in the game world, a secret organization known as The Red Brigade. One of the special concerns of the Red Brigade was the repeated kidnapping, murder, and robbery of members of the Avengers. The Red Brigade members weren't even very high level, but they kidnapped and ransomed even some of the higher level Avengers successfully because they had numbers - lots of guys with crossbows and nasty poisons. For YEARS, both in real-time and in-game, the Avengers searched the world trying to find the leaders of the Red Brigade but repeatedly failed.
And players DESPISED this. To have their PC killed and all their extremely valuable equipment stolen, and then have to pay a ransom for the corpse - and then pay to get the corpse resurrected! Eventually, one or two players deduced - from meta-game knowledge - who led the Red Brigade, but to their credit kept completely silent. But one day, as was common, the Avengers were making use of a Commune spell from the highest level PC - a 20th level cleric. But, the information we had been seeking took up only a few questions leaving a lot of questions unused. Not wanting to waste them we asked along several other lines of questions, one of which was the Red Brigade.
As fate would have it someone FINALLY managed to ask the right questions, in the right way, put two and two together and it was revealed that one of the wizards in the Avengers (named Bored Flak) was in fact a traitor and the leader of the Red Brigade. I say, "as fate would have it," because it was a COMPLETE FLUKE that Bored Flak was not present with the others at the time and thus remained at large and remained a PC/NPC nemesis of the Avengers to the end of the campaign.
The revelation however STUNNED everyone and after much shouting of, "I DON'T BELIEVE IT!" we all stood and shook the hand of the player of Bored Flak for such a masterful job of pulling the wool over our eyes for years, and driving us all nuts.
[Another best-game-experience-ever was when the Avengers came to a standoff against EACH OTHER at the launch console of an Intercontinental Magic Missile Silo. But that's another thread...]