The function "update_instance()" in class assignment_base, file mod/assignment/lib.php, is documented as follows:
* Updates a new assignment activity
* Given an object containing all the necessary data,
* (defined by the form in mod.html) this function
* will update the assignment instance and return the id number
* The due date is updated in the calendar
* This is common to all assignment types.
* @param $assignment object The data from the form on mod.html
* @return int The assignment id
However, in the actual code, it does not return an assignment id. Rather, it always returns "true".
This doesn't seem to cause malfunction, but it caused me some headache when developing a custom assignment type...