So I'm riding the Mavericks, I mean I'm using OS X 10.9.2. :)
I played around with the account settings and now the account is damaged.
Below is exactly how I damaged it.
I wanted to change my username.
According to http://support.apple.com/kb/HT1428, I first created a new account, then went to the directory "/Users" and changed the name of the folder "/Users/mac" to "/Users/Air".
Then I went into System Preference -> Users & Groups, and entered the "Advanced Options" of the account named "mac".
I happily disregarded the "WARNING".
I deleted "User ID".
And deleted "Group", which automatically turned to "wheel" after the cursor was moved to the next field.
I Changed "Account name" from "mac" to "Air", and changed "Home directory" from "/Users/mac" to "/Users/Air".
I clicked "OK", restarted and logged in the newly-named account. Then it says the resource library was damaged, and the system stopped responding (that beachball spinning!)
Any help is greatly appreciated!