Edit the settings for a questionnaire and set the 'Grade' to 'No grade', then set the 'Completion tracking' to 'Show activity as complete when conditions are met' and tick the 'Student must receive a grade to complete this activity'.

When you save the form, you receive the following error:
An internal error occurred in the completion system. (System administrators can enable debugging information to see more detail.)

Debug info: Cannot find grade item for 'questionnaire'
cm '104' matching number '0'
Error code: err_system
Stack trace:
line 1256 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 821 of /lib/completionlib.php: call to completion_info->update_state()
line 388 of /course/modedit.php: call to completion_info->reset_all_state()

Obviously, this is a silly combination of settings from the end user, but it would be nice if there was a helpful warning message, instead of a fatal error.