EllisLab/CodeIgniter — pull request #933 by wrightlabs (2012-01-16 23:09:51+00:00)
Title: It appears the Security class has been added to the system/core folder a...

...nd is loaded automatically as well.

Using $this->load->library('security'); in controllers currently returns FALSE, yet you might not notice because the Security class is already loaded.
I discovered this because of a custom Loader class I was using returned an error when loading the Security class.