I am testing the cart with Paypal sandbox, and on returning to the site after succesful Sandbox payment, the wrong message is displayed to the anonymous user. The message displayed is the one meant for users who are already logged in, however I am allowing anonymous users to perform the payment and therefore the right message should be the one meant for new users showing the new user credentials etc. By the way I unchecked the box that supposedly logs in the new account automatically on successful payment. Also, if I try the payment with the credit card test gateway the problem does not occurs and the right message is displayed.

Could it be that since I must be logged into the Sandbox to do this test, confusiona arises as to the anonymous user appearing to be logged on when the user (me) is only logged on at Paypal?
This looks definitely like a bug to me.