A course is set up to allow guest access (with the enrolment key only).
It is my understanding that when a site user is assigned the course role of Guest by the course teacher (or above) for this course, s/he can access that course by clicking on it in the MyCourses block (and no enrollment key is required) ... s/he just has access to the course with the assigned role
Instead, the user only receives the description of the course and is prompted for the enrollment key to allow s/he to enroll in the course as a student.
Luckily ... the supplied enrollment key does not allow him or her to enroll as a student, it just recycles the page requesting the enrollment key.
But the assigned guest user has no access to the course.