Код:
An object dispatches a SecurityErrorEvent object to report the occurrence of a security error.
Security errors reported through this class are generally from asynchronous operations, such as loading data, in which security violations may not manifest immediately.
Your event listener can access the object's text property to determine what operation was attempted and any URLs that were involved.
If there are no event listeners, the debugger version of Flash Player or the AIR Debug Launcher (ADL) application automatically displays an error message that contains
the contents of the text property.
There is one type of security error event: SecurityErrorEvent.SECURITY_ERROR.
http://livedocs.adobe.com/flash/9.0/ActionScriptLangRefV3/flash/events/SecurityErrorEvent.html
Возникает, например, когда мы пытаемся стучаться в инет из файла, открытого локально и опубликованного с дефолтной установкой Access local files only