10.8 Notification Center occasionally freezes system
From Apple Support Communities Thread:


  
  Restart your computer and hold down cmd + R to boot into recovery mode.
  From the menu bar, select Utilities -> Terminal
  Type in "resetpassword" without the quotes.
  In the window that pops up, there will be a button in the lower right corner labeled "Reset Home Directory Permissions and ACLs".  Click on that.  It will take a moment to complete.
  When it finishes, simply restart the computer from the Apple menu at the top right.  Your computer will restart normally.