User class not found

1 advice - to prevent duplication of CI Class names try adding prefix or suffix to your class : Utilities_library..

This should only be done if you want to extend native CI-Libraries.
https://ellislab.com/codeigniter/user-gu...aries.html

Having looked into the sourcecode of Common.php, I have the impression that there is a bug.

Just a note: both user guides versions can be found here now on the codeigniter doc site

http://www.codeigniter.com/docs

There's only one rule - please don't tell anyone to go and read the manual.  Sometimes the manual just SUCKS!