If you use the Add Participants tool and click continue without adding a username or guest address, Sakai will give you an error:
"Please enter username(s) or guest email address(es) to add to this site"
The error lacks punctuation and also doesn't match the style of many (most?) other errors encountered in Sakai. Most errors start with "Alert!":
Alert: Please enter a valid Username for the instructor of record.
Alert: Please specify the assignment title.
Alert: This assignment has no instructions. Are you sure?
Alert: The site has to have at least one user with role Instructor.