I tried a Software Update today, found a couple of items and decided to install them — but on download I got an error message that read “none of the checked updates could be saved” and “you do not have appropriate access privileges”!
Focusing on privileges, I figured I must do a Disk Repair on my privileges. No go. What my system was trying to tell me was that folders for these updates already existed in my Library/Updates folder — and that I didn’t have permission to overwrite them. I had to go in and delete these folders manually (my index.plist didn’t have to be deleted).
When I ran the updates again it worked. Lesson learned.