As researcher, I should know this: Keep track of your stuff, know where stuff is, know what you do and do not have, what's yet to be done, etc.
I end up spending a week or so working with a Polaroid that I assume is the 220. In fact, I've put images into sets and tagged things on flickr based on that understanding. I'm in the middle of packing to move and as I was going through a cabinet of cameras, I come across another Land Camera. I'm puzzled because I know I have a broken 225 that I began modify to be a wax paper film plane camera, but I know this isn't it. The modified 225 looks much different and I know that's in another closet. So I open up the recently found camera and it's the 220. Turns out I had been working with the 100. Hmmm. The kicker is that I don't remember buying this. I vaguely remember waiting for a second battery to arrive, but I thought maybe I was just ordering a spare to carrying with me for when I'm at antique or resale stores.
Moving has been a really good excuse (or incentive) to start an inventory of what I have. I'm not at the point (not yet anyway) where I've accidentally purchased a camera that I already have but I can see things going that way at some point. Of course, I've often purposely bought a second (or third or fourth) camera that I already own because I've wanted to make sure that if anything happens to the first one, I'm covered.