Topic: [BUG?] Cannot log out when you are banned.
If you are logged into a banned account, you cannot log out unless you delete the cookie.
In login.php, 'include/common.php' is executed before checking the GET inputs. 'common.php' executes function check_bans();, which (if user is banned) executes function message();, which finally prints the ban message and terminates the script. This way, the GET inputs will never get checked and you will never log out.
I don't know if this is intentional or not. If it is, I can see your reasonings and I agree with them, but you should make that optional.