That is one of my biggest fears in doing any mechanical work which requires disassembly. In addition to the traditional surgeon's sponge count ("did we leave anything inside the patient?"), I want to make sure we put back everything we took out.

My boys and I were changing the timing belt on my wife's 2001 Passat wagon, and we almost forgot to replace the water pump, which was several years older than the timing belt, a known point of failure on these cars, and our main motivation in doing the timing belt job somewhat early. Fortunately I noticed before we had reassembled everything.

I just finished replacing the drain pump and cleaning out all of the accumulated gunk in our four(!)-year-old Whirlpool Cabrio washer, and I had reassembled almost everything when I noticed that I had left the cover off the inner tub's drain hole. I am now pretty fast on teardown-and-reassembly on this thing, as well.