Thanks to Matt Clarkson for reporting this.
If activity completion is set to require a grade and the following settings are made:
show activity is complete when conditions are met.
require grade

we get this error:
Debug info: Cannot find grade item for 'scorm'
cm '284' matching number '0'
Error code: err_system
Stack trace:
line 1226 of /lib/completionlib.php: moodle_exception thrown
line 584 of /lib/completionlib.php: call to completion_info->internal_systemerror()
line 517 of /lib/completionlib.php: call to completion_info->internal_get_state()
line 692 of /mod/scorm/lib.php: call to completion_info->update_state()
line 153 of /mod/scorm/lib.php: call to scorm_grade_item_update()
line 447 of /course/modedit.php: call to scorm_add_instance()