Aquifer disaster #1:
A botched attempt to get through an aquifer by channeling a load of magma into it... which resulted in half my fort getting flooded with magma.
Aquifer disaster #2: Not very serious, but frustrating, until it yeilded some interesting advantages:
I was trying to pump out an aquifer, and was doing okay, but two of the dwarves decided to take their babies (and some puppies) to the building site, who then got separated from their parents/owners, and suck in a couple of tiles that had most of the water pumped out of them, but were surrounded by deeper water, preventing them getting out or any rescuers getting to them.
I spent a long time trying to get them out, but was eventually distracted by more serious matters (a goblin seige).
After the goblins had attacked my fort (and slaughtered most of my best troops, who had all decided to rush to the one part of the fort that the goblins could shoot into, despite there not being any way for them to actually get in there), the goblins decided to go after the babies and puppies that were trapped in my aquifer-excavation site.
Well, they got in, brutally murdered the babies and the puppies
...
And then realized that they were trapped as well.
So, I opened my gates, let my dwarves out, and told them to dismantle the axel that provided power to the pumps...
Cue 9 drowned goblins
Karma is sweet