2.6, 2.6.1, 2.6.2
If you edit a course and change the shortname or idnumber to a string that is identical except changing the case on some letters then Moodle will display an error to the user saying the name or idnumber is already in use.
1. Create a new course
a. Full name - "My Course"
b. Short name - "My course"
2. Complete creating the course
3. Edit the course
4. Change the course short name to "my Course"
5. Save the changes
6. You should get the following error
Short name is already used for another course (my Course)
More information about this error
Error code: shortnametaken
line 2588 of /course/lib.php: moodle_exception thrown
line 152 of /course/edit.php: call to update_course()
7. Repeat for idnumber
Since the course with the duplicate name or number is the course you are trying to update I'd expect this to complete.