go to Admin-> (Users Section) Add a new User.
Enter info about a new user but... make mistakes like... write an email address already present for a different user.
Submit the form pushing the button Update Profile.
When you get the same form with some extra red warning regarding your mistakes, ignore them and exit the form i.e. returning to your Moodle home page (throught breadcrumb).
Now go to Admin-> (Users Section) Edit User Accounts and... one more empty user is on your list.
This means that when you push the Update Profile button Moodle creates the record before checking its integrity. This is an error. I don't want empty records in my moodle tables.