I’ve occasionally had an error when installing software that goes something like “The update could not be saved… You do not have appropriate access privileges.” I’ve tried deleting the software update caches, fixing permissions, etc., all to no avail.

Well, this Apple discussion thread had the answer. You just have to delete a folder with the same name as your update from /Library/Updates. Or just delete everything, I suppose.

I had to delete the offending folder from the terminal using “sudo” for some reason. I guess it was set as root-owned or something. Weird.