Conflict between 2 CourseDescription classes
The CourseDescription class is defined in main/inc/lib/course_description.lib.php and main/coursecopy/classes/CourseDescription.class.php
In some cases (like when calling the constructor for the CourseDescription class in a plugin, the code expects the call to include parameters (as in the class defined in coursecopy/classes) whereas we want the constructor without parameters (as defined in main/inc/lib).
Define a namespace for both to avoid conflict. If change can be made to only one of these, I prefer to make it in the coursecopy folder as this is less used. Make sure you modify all corresponding references to use the namespace.
I found this error when using the session_slider_block plugin for the tmi branch (call in index.php, line 33). Not always failing, though.