Last week I tried to install Yosemite onto an external drive that I had in order to take a look at it and try it out. The installation went great, my MBP rebooted, and I was looking at a new OS on an external disk. I played with it for a few minutes, decided it was time to do real work, and shut down my system. I unplugged the external drive, and when I rebooted my MBP, Yosemite began to install on my MBP as my main OS. I thought, "Surely this can't be happening," but after a few minutes I was staring at Yosemite on my main computer with no recourse because I had, I though, taken all of the proper precautions. It's not a huge deal, but I wonder how many other folks this might have happened to, or if there was some major step that I missed completely that I need to know about for next time.