The time the party got pwned by a bard.
It was a megadungeon game, and for a bit of spice, the DM created a rival party exploring the dungeon as well. This party was led by Sir Meriwether.
As the campaign progressed, it became very clear that we were not just competing for the dungeon’s treasures, but everything else, too. When we decided to build a clubhouse and become a recognized Adventurer’s Guild, Sir Meriwether decided that he had to have a church. In order to gain recognition for the guild, we had to appease the town matron and court one of her granddaughters. Sir Meriwether decided to come courting as well. And he had his Charisma and social skills boosted out the wazoo. We won the granddaughter, but only barely.
This whole time, we assumed that Sir Meriwether was a paladin and even though he was being a dick and trying to secure every resource that we were after (and won them more than we care to admit), he was lawful good and had good intentions. Our first clue otherwise is when we find out that he was carrying a holy symbol that was also an amulet of undetectable alignment. (He had actually indirectly commissioned our wizard to make it for him.)
Armed with this knowledge, we do some digging and find out that he’s not actually a paladin (fallen or otherwise), but in fact, a bad guy. Then the gloves came off, though my wife and I argue about which of us actually struck the killing blow.
Once he was defeated, the DM then let us behind the screen a little bit and revealed that he was actually a neutral evil bard. Everybody’s jaws hit the floor at once. The more I thought about it, the more sense it made. The boosted social skills became much easier for a bard to pull off. He wore mithril so he could look heavily armored and still cast his arcane spells unimpeded. He could even fake lay on hands because bards have access to cure spells (this was 3.5).