As noticed while testing MDL-62493, see comments there: https://tracker.moodle.org/browse/MDL-62493?focusedCommentId=622750&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-622750

1. Create a course that does not have guest access and a user who is not enrolled in this course
2. Log out
3. Click on the course name, you will be redirected to the login page
4. Login as a user who is not enrolled
5. You will be redirected to enrol/index.php that says that you can not enrol yourself
6. Click "Continue" - you will get an error that you are already logged in.