My latest tale of development:  I thought I had figured out how to finally get my current animation and depth issues solved.  I just needed to do it.  So, I started out by adding some new methods / code to one of my classes.
Then, a few days later, I went to use the new stuff.  It wasn't there!  Searching the entire project, nothing!  None of the new changes are there.  I jumped to a command prompt and discovered that I had apparently reorganized my source tree at one point, and now had TWO copies of the files.  I had made the changes to the wrong files -- some old ones.
Well that was silly, but I could at least copy the changes into the proper files, so no harm done.  It turned out there were a few other files that I also had duplicate copies of in my source tree, again because of reorganizing in the past.  No big deal, I'll just use a command-line wildcard pattern to delete the other old files that were NOT modified.
Oops, I used a pattern that was too generous, and deleted the newly modified files too.  The ones that I wanted to copy code from. 
 
......of course, since I'm paranoid and have a ton of different versions of my source tree backed up in different places, and had made a fresh copy before my latest time poking at the source tree, I was able to easily restore it.  It was still a bit scary, though.  Backups are important!