It seems that when creating a new user, it's possible to set the username to an existing user email address which will then cause the existing user to not be able to log in.

For example, a super user with username admin and email address of admin@example.com creates a new user with email user@example.com and accidentally sets the new username to admin@example.com

This prevents the super user from being able to log in to their account using their email address.