One of the more obvious issues with modern software is that system complexity has evolved more quickly than system repair and recovery mechanisms. The rules of biological evolution don't fully apply, at least in the near term, to computer systems.
Multi user accounts, on both OS X and XP, illustrate this. Today my Firefox update failed with a cryptic message on my XP box. It said a file was in use and could not be replaced. I checked my processes and didn't see anything. Logging out didn't help. Downloading the full executable and reinstalling didn't help.
Right. Firefox was running in another account, indeed the Firefox Update was running in the other account -- and it was stalled there too. The Firefox installed Google toolbar process was running in both user instances as well.
Ideally the Mozilla installer, or the underlying OS, would detect this state and provide a meaningful error message.
One day ...