Prerequisite:

• Comments are enabled in moodle on system level.
• There is a course on http://MYMOODLEDOMAIN/course/view.php?id=COURSEID with comments block added
• Some comments have been added in the comments block

In the comments block, there are links to each commenting user's profile:
http://MYMOODLEDOMAIN/user/view.php?id=USERID&course=COURSEID

Now, if the commenting user isn't enrolled in the course because he has unenrolled in the meantime or because he had the capability to comment without enrolling (Admin, supervisor), the link to the profile will result in an error message "This profile is not available because this user is not enrolled in this course."

Wouldn't it make sense to hide the link and show only the user name in this case?