If the user tries to log in repeatedly with incorrect credentials, eventually they will be locked out. They will see the following error:
"Your account has been blocked after multiple consecutive login attempts. We've sent you an email with instructions on how to unblock it."
It looks like this on the login screen:
They will also be sent an email that looks like this:
Clicking on the “UNBLOCK” link in the email will unlock their account. They will be presented with this screen:
and they can reload the Signals Notebook URL and try logging in again.