A few months ago, a friend of my wife bought herself a Firewire drive for her iMac (the original candy-colored model) and wanted my help getting everything set up. No big deal: I moved her home directory to the Firewire drive, and everything worked fine for months. Then, for no apparent reason, her home directory disappeared. It was like she had created a whole new account. She could still see the Firewire drive mounted and it still had all of her files, but her iTunes and iPhoto were empty.
Last night, she cooked dinner and I investigated. Turns out, some Firewire controllers are known to "disappear" on occasion. I downloaded a firmware upgrade from LaCie, so hopefully this problem won't happen again in the future. But then there was the damage control. Apparently, since I had earlier set her home directory to /Volumes/<external drive>/Users/<username>, but the drive wasn't actually there at one point, the Mac helpfully created the directory. Once the drive came back, its original mount point was taken, so it got assigned to /Volumes/<external drive> 1/, which you could only figure out from the command line. The Finder showed the external disk, exactly where it was supposed to be. It took me a while to realize what was going on because 'pwd' was telling me I was in the right place, but 'df' said I was on the internal hard drive.
I'm sure there's a moral to this story somewhere that somehow combines the value of computer nerds and good cooking with a sidebar on outsourcing and the importance of continuity of administration.