From the Delegated Admin console, admins can create new objects, such as Users. This will pop up a new user creation form, where the admin fills in desired User attributes, then clicks a blue "Save" button at the bottom of the form to create this user.
There is a delay/lag, especially when having User onCreate scripts, from when the "Save" button is pressed, to time the form is dismissed and the user successfully created message appears in the bottom left corner. This delay is causing end users to press the "Save" button more than once, triggering some user experience issues.
What typically happens, is that an admin fills in the form and presses "Save". During this time the program is creating the user, but it may take up to 4-5 seconds for the form to actually dismiss. So the admin believes they haven't created the user, and they press "Save" again. Since the first button press is successfully creating the user, by the time the second button press occurs, the user already exists and the form now displays an error on the username field as it is no longer unique.
If the "Save" button were to be disabled after the first press, until a user is successfully created or an error message is displayed to the admin, it avoid this "double-click" scenario.