While playing with the enrolment UI (enrol/users.php) I've found various inconsistencies:
1) The whole "roles" cell acts as a button for the add roles popup.
2) If you click 10 times in the "roles" cell, without picking any role, and the 11th time you pick one role, it is added (at least visually) 10 times. (screenshot added)
3) The "add" roles popup is not modal (aka does not dim the UI), it continues allowing other actions like "remove role" to happen (screenshot added).