Today, after upgrading one of our machines to the latest security update, the iMac G5 stopped booting. Starting the machine in Single User mode (command+S) revealed that the mDNSResponder daemon was crashing and restarting in an infinite loop.
'The Mac was stuck in a loop trying to execute mDNSResponder (which is the basis of Bonjour networking) and the system wasn't successful.
On the Apple forums nobody is able to pinpoint the problem yet, but it looks like it is a pretty common disease. If you do not need Bonjour, this is what solved it for me:
