The history of this trap construction is full of mistakes and time waste. Here are some:
-At start, the lenght was nearly one third of what it is today! The trap was working, but I had a hard time catching more than a couple gobs without them going trough before the gates were closed.
When I understood I would have to greatly extend the lenght, I got depressed by the huge amount of work needed. The first part was hours and hours of work, and I had to do more than double that! That nearly put a definitive stop on it, and it took me 6 months to finally start working again, and a few weeks later, tadaaa!
-Once, after testing the trap and loading in with lava, I forgot to forbid the door leading to the reservoir. Of course, while I was not looking, a dwarf working on the pumps at that level opened it (what for???) and burned alive. The side effect was that the spilled lava blocked the way and two more worker nearly starved to death.
-The most awful huge astounding horrible mistake I did was when I installed the pumps. I had already tried the stacking method, so I knew what I was doing. Copying what I did before, I designed the walls, floor, for hours, then put the pumps, with my fingers sores from the repetitive task done hundreds of times.
Then I test-fired it. And nothing happened...
I realised I had forgotten one little thing... to put a hole under each pump so they transfer power to each other! And removing the floor afterward did NOT repair it! I had to deconstruct and rebuild EACH pump!! And the worse is, I already did the same mistake when I built the first part of the trap! I KNEW this mistake, and I did it again! Aaarghh!
-Of course, I had numerous case of stupid dwarves blockinf themselves when deconstructing floor, building wall that prevented the construction of other pieces of wall, etc... I also had some cases of cave-in, the most stupid one was when I deconstructed a floor, and realised that floor are not linked to bridges... So I had half a dozen square of floor, adjacent to a bridge and nothing else, falling throught the iron floor grates, the bottom cistern, and the workshops under it...