I'm not sure what happened but I don't seem to be able to log into the guest account anymore.
To save seats I banned the guest account a while ago. Well I would like to enable it again so I unbanned it. When I go to the login page it doesn't show a little link to connect as guest (I believe this was displayed before?) and either no username or using 'guest' with no password doesn't let me login. It has the message Can't authorize you by the given credentials.
I tried disabling, saving, re-enabling, then saving again in the Administration > More Settings > Global page. It added the global observer permission to the guest account and that appears to be all.
I have the LDAP auth module enabled if that's important.