The first time I disassembled and rebuilt a carburetor, the results were not impressive. Had two tiny pieces left over - never a good thing.
I recently bought a Dan Wesson 22, and decided to replace the springs. Unlike some guns out there, I could not find much documentation on disassembly, although I have an older book with a few pages of instructions. Anyway, I took it down, and replaced the springs. Putting them back together is always the hard part. Took me about an hour, going slow, fitting parts, getting the springs in. I get to the end, and the gun will not function in double action.
I open it back up, and watch the function and see the DA sear keeps slipping under the hammer. I think to myself, there must be a something to keep it from doing that that I missed. Well, I look around the box in which I do all the work, and sure enough there is a very small pin and spring that the instructions never mention (although it is in the schismatic I have) and that I did not even see fall out that keeps the sear in the right position--and, of course, that is the first piece to be reassembled, meaning I have to completely strip it again (well, almost, as I did not have to remove the mainspring) and reassemble it.
The good news is that, because I have already reassembled it once, reassembling again took only 1/4 of the time, since I now knew the process.
All in all, a great experience.