This one is a bummer when it happens.
Our security systems only allow a single instance of any given username or email address. So, if you have a typo in your email address when you create an account, you'll have to create a new one with the correct email address. However, since you already used that username that you really wanted for the first (incorrect email) account, you will have to choose a new username to go along with the proper email address.
Bummer, we know, but security is a bummer sometimes too. At least you are the only one that will see your username.