I made my first large death trap. When the first lever was pulled, it would seal the large entry hall off with floodgates. The second lever closed the drainage system, while the third and final lever controlled the water intake. The intake was hooked up to a river (though it had to travel quite a ways to get there), while the drain went down a long tunnel until it reached an automatic pump which was still under construction. It worked, sort of; I was able to drown elves and dwarves (for Fun, mostly). The flaw to the trap was that it took so long to fill with unpressurized water that the pack animals died of dehydration before they drowned, even as the water reached around 4/7. Then I found out the lever that controlled the intake was missing. Completely gone. So like ... I had a sealed room, ever filling with water, and a broken drainage system. Needless to say, I had to abandon eventually.
The second disaster fort I made started off incredibly; it had copious amounts of ore, coal, and gems, guaranteeing a good metal industry. Huge forests and a running stream. Skeletal giant eagles (which thankfully didn't notice me at all). Well, I added a cistern and it broke the fort; essentially, I died to water pressure. My main level was located 1 z-level under the river. The cistern was two 2 levels tall, and a few floors below the main. I built a small meeting area above the cistern, with a hole in the floor, acting as an elaborate well. Well, apparently water pressure can go down, over, and then UP. The room happened to be connected by ramps, and ended up flooding the entire fortress; workshops, stockpiles, bedrooms, everything. I managed to save my dorfs from the bedrooms by tunneling down from above and making stairs up, and mining out adjacent rooms, but I shortly abandoned afterwards.