My favorite from DnD was probably our DM's least favorite.
The adventure started with a deep dragon sending us on a quest that had something to do with a high level wizard. At the end, we found out that the wizard had double crossed us and nearly gotten us killed. Now, our DM later revealed that he intended for us to confront said wizard, and through some minor quests, learn that the dragon himself had set the entire thing up. However, things kind of went in a completely different direction.
I spent the entire week before the next adventure figuring out strategies for taking down the wizard. When the game started, instead of verbally confronting the guy. we jumped him 4 on one! In the middle of the fight, I snuck away into the wizard's tower, found a room on an upper floor full of vials, jumped out the window, and launched a fireball into the room.
The explosion was rather impression, and our friend the wizard got caught in it. As character's sister caught my elf, the other two killed the wizard (I should note that we're all single-digit characters).
The DM suddenly got very quiet, and we anticipated a 'Rocks fall, Everyone dies' moment. Instead, he told us what was supposed to happen, then shrugged his shoulders and had the deep dragon show up.
Since we were still pretty beaten up from fighting the wizard, I told everyone to lie among the debris and play dead. The DM determined that the dragon would show up and poke each person, and they would make a Wisdom roll (He hated saving throws) to stay still.
Well, obviously, the wizard didn't move. Next is was the first warrior (the only character with both an Intelligence and Wisdom below 10), who passed easily. Then the second warrior (average stats for everything but Wis), who also passed. Then my elf, who passed easily. Finally, the cleric (also my elf's sister, who had a 19 wisdom), who FAILED, leaping to her feet in a defensive stance directly in front of the dragon.
Groaning, we quickly formed our strategies, knowing we had another fight on our hands. My elf, who was still lying below the dragon, launched several fireballs into its stomach. The two fighters then attacked it from behind. Then the cleric used whatever spell she had, and after another 30-minute battle, we won again.
Our normally jolly and happy-go-lucky DM just stared at his papers for a long moment. He then opened the Monster Manual, stared at it for a good five minutes, and declared that 2 red dragons, servants of the deep dragon, had shown up to avenge their master.
I don't remember what happened next except that the battle lasted about an hour in real time, and somehow we won again. After the second dragon fell, our DM just shut the book and declared the quest over. He would later joke about it, but that night he was less than pleased. I was asked to again take over as DM after that.
Although we still gloat about the game, I believe deep down that luck was definitely on our side.