MetaMorph X is a program which allows you to alter, via ‘themes,' the look and feel of the Macintosh system interface, not unlike what Kaleidoscope provides to Mac OS 9 users. However, it has the capability of causing great damage to your system under circumstances that can very easily befall any user.
If you install MetaMorph X, YOU MUST MAKE SURE that the theme you use is for the current version of your operating system. You cannot use a 10.0.4 theme on a 10.1 system, for example! If you have MetaMorph X installed and are installing a system update, MAKE SURE it is completely uninstalled and that you are in the "Aqua" theme that Apple packaged with Mac OS X before installing any update.
If you don't, what will befall you?
The next time you try to log back in, you will find that your system no longer presents you with a login window. (Enabling automatic login prior to theme installation is not a solution.)
If this happens, the only solution so far has been to re-install OS 10.0. The problem with that is that after the install of 10.0 is complete, many systems will experience kernel panics immediately at startup. Fortunately, you should be able to continue the upgrade to 10.1, which should cure the panic. (Fortunately, even though my computer was "panicking," the Upgrade CD still recognized the system as being eligible for an upgrade to Mac OS 10.1 — the disk was not greyed out. YMMV.)
According to a post in the MacFixIt Forums, the reason for all this sturm und drang may be that the Extras.rsrc file gets corrupted, and that this may be a bug with Apple's themes implementation.
[Editor's note: Whenever you decide to use something that modifies the lowest levels of your system, please make sure you backup your data first! It's just generally good advice in case you end up reformatting your drive.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011130160012247