Best Campaign surprises!
My best campaign surprises have all been times when I started doing something that seemed, initially, meaningless or suicidal, and ended up shocking the whole party. But that was always as a player...
--I once collected the bounty on my own head in a Star Wars game. I also disabled an Imperial corvette that was chasing down the captured bulk freighter we were in by loading up an escape pod with anything and everything explosive I could find and launching it in the middle of the swarm of other escape pods that were abandoning ship. The pursuit ship pulled it into the landing bay just before the timer went off...
--Different Star Wars game, playing a Jedi Master, we came across an AI of sorts who had not only synthesized a flesh body to inhabit but had also come up with a personal shield that could withstand even lightsabers (based at least in part on the same technology...the GM played fast and loose with the specifics and we rolled with it). So my Jedi used Absorb/Dissipate Energy to sap the shield while another Jedi took out the AI (who was trying to basically create the Star Wars equivalent of Berserkers, for those who remember their Fred Saberhagen novels). It shocked the GM on two levels...not only did he not expect that particular tactic, but it was pretty much the only time those two characters were on the same page about ANYTHING...and neither of us told each other what we were going to do, either IC or OOC (this was a tabletop game, so the fact that she acted immediately upon me creating the opening was surprising, even to me...I figured she would act after a round of realizing what was going on, but she had apparently already decided that shield or no shield, she needed to take action.)
--A GM thought he'd be clever and tease the party with a legend about a lost runesword. We found it...turned out, it was a sword for a gnome. Most of the party declared it useless as it was only slightly larger than the knife in your average old-school Clue board game...but my bard thought it was a fascinating trinket and would make great fodder for storytelling. So he wore it as a pendant on a leather necklace...and thus, having claimed it, gained access to all its additional magical abilities. (Same bard also stayed behind in a pocket dimension because two gods were going to fight each other and he wanted to be able to tell the story...)
--Same GM, different game, had us fighting a minotaur lich. The only weapon I had with any magical abilities to speak of was a quiver that magically created Chaos arrows (you'd fire and if you hit, you rolled for a result on a table...anything from inflicting massive damage to causing flowers to erupt in the area...) Third round of what was supposed to be a massive, drawn-out battle that would have brought most of the party close to death, I finally managed to hit...and the result was that the arrow healed all damage done. I was initially outraged that I'd just undone all the success of the rest of the group...it wasn't until I noticed the GM looking down at his character notes in shock that I thought to question what had happened...healing the undead like that kinda makes it hard for them to stay undead, it turns out...
--Had a GM who thought he was going to overrun the party with kobolds in a Palladium fantasy game. My troll (who specialized in improvised weapons) decided the closest, most dangerous thing he could get his hand on was two of the kobolds...when they were no longer effective as clubs, he threw them away and grabbed two more...not a campaign surprise, by any means, but the look of shock on everyone's face was priceless.
Those are just things that I did. The groups I played in through college and for about five years afterwards seemed to specialize in finding weird and unique ways to twist the situation back on itself.
The biggest campaign surprise that was ever sprung on me was in a Star Wars game that we had been playing for years...the GM decided to play with some alternate reality options (which he'd done before...but those had been 'butterfly effect' kinds of things, where we changed the outcome of one of the events in the movies and it ended up totally shifting a timeline to where not only did the Empire win, but my character was a hero of the Empire...we had to find some way to change things back...or rather, I did, because I was the only one who got to see what the outcome was...) Anyway, in this particular reality shift, my die-hard "No such thing as the Force and the Jedi are all delusional" bounty hunter (who, by weird flukes of the dice throughout the years of playing him, never once actually saw anyone successfully do anything with The Force and was actually the one responsible for a few aspiring Jedi types THINKING they had done something with the Force) suddenly became a member of the Jedi Council, who had been a padawan to Anakin Skywalker (who never turned, in that reality, to the Dark Side). He was also the only one in the party, ironically enough, who had no recollection of his 'former self' (another vagary of the dice), but he could tell that everyone telling him about 'who he used to be' very firmly believed it. This was even more of a shock because I had to work the night that this happened, out of town, so when I was done with work, they called me to ask a few questions to help determine how this would all turn out.