OSX has a rather agressive NIS authentication mechanism. Once configured, if configured wrongly or the NIS server is un-available, the OS can get horribly locked up trying to contact it. This can be a pain, as it even prevents going in and changing the config. Bit stupid really. Marcel Bresink has written a very comprehensive set of instructions on using NIS with OSX, including (at the bottom) a way of fixing a mis-configured machine.