My first fort was built step-by step according to the tutorial on the wiki. When the tutorial ended, I decided I wanted a well. A mined out a nice little room next to my dining room (which was also right next to my kitchen, still and food stockpile, and the main meeting hall). I took precautions by digging the well in the winter when the river was frozen, and building floodgates as a safe guard. I even had a final failsafe with a set of "airlock" doors controlled by lever into the well room in case I needed to seal the whole thing.
Spring comes, the river thaws and begins filling the cistern. So far so good. As the well is almost full, I give the order to throw the lever to stop the flow. Mistake number 1: Floodgate lever was directly next to the well. By time someone actually got around to throwing the lever, the well was overflowing, washing anyone who tried to pull the lever into the well and drowning them. No biggie, I lost a few dwarfs and a cat or two, but the airlock will save the fortress. Mistake 2: Airlocks don't work if both doors are set to the same lever (I did not realize that levers work on a binary on/off thing, instead I thought they toggled everything connected to them as they were pulled.) Also, the airlock level was inside the airlock itself (the idea was for a dwarf to go in, pull the lever to close the door behind them and open the door in front of them and vice versa). The airlock is stuck open and before a sacrificial lamb can be called upon to seal himself and the water, the lever floods above dwarf head height.
So now my dining room is slowly flooding, but I'm optimistic because the dining room has doors that will trap the water and I'll just have to make a new dining room. I hastily tell my people to deconstruct all the furniture in the dining hall and hollow out one of my mining tunnels for the new dining room. I designate a new food stockpile and tell them to start making new kitchens and stills and start smoothing the room. About the half the tables and chairs are deconstructed (but left where they stand) when I start to get mass job cancellations due to unpassable terrain. Okay, well, I can always make new tables and chairs. I lock the doors to the old dining room and continue on (a few more cats drown, but whatever). Eventually I notice all the dwarves are moving very slowly and flashing with blue arrows. Turns out that creating a new food/booze stockpile doesn't mean the old one ceases to exist, so no one bothered moving my 2000+ units of booze out of the flood zone, but they still counted as "in the fort" so the stock screen couldn't tell me everyone was dying of thirst (it was winter again by now, so no water to drink). Also, everyone was getting really sad because they didn't have any booze, so the tantrums started. Once half the fort was throwing tantrums, a Cyclops stomped onto the map and that was pretty much the end (a tantruming dwarf deconstructed my main drawbridge, so the cyclops had an unimpeded path to wreak havoc throughout the fort).