Perhaps this applies too to 1.9 and 1.7. To reproduce in 1.8:
1. Enrol a user in a course with unassignself capability enabled
2. Impersonate the student or login as such student
3. Go to his/her personal profile
4. Click in the "Unenrol me from XXX" button
You get a "Sorry, but you do not currently have permissions to do that (Assign roles to users)" message.
This problem can be resolved by modifying user/view.php or by modifying course/unenrol.php. I've attached a patch for 1.8.1+ that applies the former solution. I you can review...
Thanks in advance