After running the latest security update on 10.4.11, I noticed I couldn't change any settings in Network Preferences. Opening up that settings screen would result in a "Your network settings have been changed by another application" window popping up in an endless loop.

It turns out the security update makes some changes to the way PPP passwords are stored. Now, instead of being stored in a world-readable file, they're stored in the Keychain. For some reason, the update gets confused with existing PPP passwords and gets stuck in a loop. The easy way out: open Terminal and delete your existing network configuration. (You might want to make a paper backup to ease the task of entering them in again later.) In Terminal, type these commands (press Return after each): You can now set your network preferences as necessary without entering the endless loop. I found this fix in this thread on the Apple Discussions site.