It appears that if you try to register with an existing username or email address, the membership registration form (shown with the shortcode) wont show an error explaining why it wont let you continue.
this is what I did :
tried to do a registration using the page that has the registration form shortcode on it. I was trying as a regular non logged in user but I had forgotten to activate a subscription and make it public as super admin (I was using chrome as logged in admin and firefox as logged out user) so when I got to the next screen asking what subscription I wanted, there was no choice (as it should behave).
After I had activated the subscription option and made it public so it could be selected, I again tried to register as a normal user using the same details as I had before but, it just redirected back to the same registration form. If I then changed the username and email address I was registering, it allowed me to go to the selection page.
one more thing..
After I had arrived at the paypal page, I received the email from the site telling me of my new username and password even though I hadn't yet paid. Is this normal? I would have hoped that the user account would not have been created until the paypal payment had been authorised.
(I have the site set to not allow registrations)