writes "heise has an article in it's security section claiming that the OS X 10.5 firewall breaks rograms that check for themselves if they were modified.
First paragraph: "Maybe it wasn't such a bad idea that Apple decided the firewall in Mac OS X Leopard should be deactivated by default. It turns out that it signs programs retrospectively in the "Set access for specific services and programs" settings — and can affect program integrity as a result. Complaints have been coming in from users that VoIP application Skype and online role-playing game World of Warcraft no longer work after they have been activated in the firewall."
Read for yourself here. No worries, it's in english :)"