Using the Wordpress Membership plugin:
When a new member signs up the process is:
Step 1: - they enter username, password, email and click next.
Step 2: they select their Subscription.
However, if they don't proceed with Step 2 and leave the website, their username, password and email is still setup as a Wordpress member (set to inactive). The problem is that if they return later to signup and enter their same username, password and email, the website will say, "that username is already in use".
So basically, visitors have one chance to signup or else they won't be able to use the same username or email address.
The solution is that the membership should not be created until the Subscription is selected and paid for.