This is a problem I found when creating a number of users. See Steps to Reproduce for information.

Reproducible: Always

Steps to Reproduce:
1. System Settings -> User Manager
2. Click 'New User' on the left.
3. put in 'caPs' as the username
4. Fill in other fields as desired (e.g Real name 'caPs', no email, password asadafaga, is administrator, no auto-login)
5. Click appy

Actual Results:  
After entering administrator password (if necessary):
[Apply] button is greyed out (disabled), but no other change are made

Expected Results:  
EITHER
New user is created with entered options
OR
Information is displayed explaining why the new user cannot be created