It's a bit of a long winded story, so just bear with me.
Alrighty, so i had set up a nice channel from the outside river to my fort, and had set up a room for farming and everything. I'd just survived the winter, and it was as if one of my dwarves felt like it was his responsibility to take over starvations job and end my game. I ordered the opening of the flood gate to the farming area, and all was going well until the dwarf's dog just had to get in the way. At the same time as the dwarf pulls the lever, his dog has a puppy, whom suddenly charges into the farm and unleashes a flood into the rest of the fort. Only one main door between the outside and my fortress prevented the water from spilling out into the wilderness. So i let the dwarves inside drown, giggling all the while, then proceeded to have the main door to the fortress removed, thinking that the water would just spell out and go away much in the same manner a cave-in acts. Much to my surprise, once the door was removed, the channel emptied the river into my fortress, and subsequently out into the wilderness. Okay, so i flooded the entire wilderness. At first thought, neat. Then i watched the flood, and it eventually started bouncing back and forth between the edges of the map and causing terrible lag. I watched it bounce around for a few minutes, then closed the game because my cpu was really crying.
Albeit very entertaining, it seems like this could be a serious problem, making accidental fort floods almost impossible to correct.