In the administration interface if you click the "new class group" button it instantly creates a "New group" Content Type Group regarding of pressing OK or not.

Steps:

1. log in administration interface
2. go to classes page (/class/grouplist)
3. click "New class group"
4. verify on URL the ID of the group that is being created
5. through URL go to classes page (/class/grouplist)
6. verify that there is a "New group" group
7. and verify the ID is the same of the one that was being created before

Update:
Even if pressing "Cancel" it creates the group