Originally Posted by NeilM
What would be more interesting would be to know why the /Users bug affected some people and not others (and perhaps why iTunes would ever set a not visible flag for that folder).
FWIW my Mac was not affected by the bug, but I'm not sure I would have noticed if it had been. Unless one uses the Shared folder I can't think of a reason to access the /Users folder.
I haven't seen anyone reporting exactly how iTunes 11.2 caused this, but the reason for why it only affected some people is known.
The bug only affects Macs which have iTunes 11.2 installed, and which have the "Find my Mac" feature of iCloud enabled. With this combination, each time you restart the computer, something causes the hidden flag to be set for /Users and /Users/Shared, and also messes up the permissions on those folders. It has been observed to occur on OS X 10.9.2 and 10.9.3, but I wasn't able to repeat it on OS X 10.8.5, which suggests it only happens if the Mac is running Mavericks.
Prior to Apple fixing the bug in iTunes 11.2.1, the workaround was to disable Find my Mac and manually fix the hidden flag and permissions of those folders (or used Repair Disk Permissions in Disk Utility).
The symptoms suggest that one of the system-level components installed by iTunes (which was running on each startup or login) was manipulating visibility and/or permissions for some files inside /Users/Shared, and due to a bug it accidentally modified the hidden flag and permissions of the parent folders as well.
I had a look inside the iTunes installer using Pacifist and noticed that it included a script which was manipulating permissions and hiding "/Users/Shared/SC Info" and "/Users/Shared/adi". That particular script would have only run at the point iTunes was installed, so doesn't explain the recurring problem. Best clue I've found: the "/Users/Shared/adi" folder is probably related to "/System/Library/PrivateFrameworks/CoreADI.framework", and that framework was the only component I could find which had a different size between the iTunes 11.2 and 11.2.1 installers, making it the most likely suspect for the bug. I have no idea what this framework does as it isn't documented, but it appears to be related to the iTunes Store, App Store and/or iBookStore.
The dependency on Find my Mac to trigger the bug is still a mystery.