The JIRA "create user" popup displays an info message that says "There are currently 10 total user(s) set up in JIRA, of which 10 are active and count towards your license limit."
JIRA counts ALL users, including "sysadmin", which in that case (supposing a 10 user license) makes it sound like the customer has reached his user limit which is not true (he can still create one user).
Note that the user can still be created, it's just that the message is confusing and should not include sysadmin in the count.