Update: This is in fact happens for both "self enrolment" and "paypal" methods.

This is similar to an issue described here: MDL-20474.

Steps:

• Authenticated user (i.e. a logged-in user not just someone who clicked the log in as guest button) clicks on a protected element (quiz in this instance) within a course (which allows guest access)
• This message appears on the screen "You need to enroll in this course before you can attempt this quiz" with a continue button
• When clicking on "Continue" the user is taken back to the course page, however it is expected that the enrollment page will be shown

The user can enroll by clicking on the "enroll" me link under the administration block, however this is not visible and confuses 90% of students

You can test this on russianasforeign.com

• Register/Login
• Go to course and click on any of the "checkpoint quizes"