Ars Magica, setting eastern Europe, a bunch of magi setting up to try to stop the Mongols from getting too far, currently worrying about the advance "scouts" getting closer to them, with unexpectedly heavy magical support of their own, pretty much killing off all the plans available so far.
At least all plans known to the group of players.
Youngest player in the group has her maga get up to the top of a mountain, overlooking a lake on one side, and the valley where the enemy troops are on the other side.
And then she uses what everyone else thought was a crazy spell she developed long before that, and moves the "lake" from one side of the mountain to the other...
The GM was like "eh, ok so the thousands of troops and dozens of shamans are drowned... Riiight...".
The player in question had from the very start anticipated where the GM would have them approach from and used the maps he had made to come up with the plan, develop the spell needed using an excuse of how it was just a bit oversized, because it was going to take the same amount of time to create the spell anyway.
And then she just annihilated the opposition that the GM thought he had secured so well against anything the PCs were capable of, in barely a moment. All the GMs plans just got thrown out, *pooof*.
Her idea of a prank.
