The potential problems that one can have when upgrading their MacBook to Leopard have been widely reported and users should be aware of them. While holding off jumping to the next big cat might be the best move for those with portables, it now looks like even upgrading to the latest version of Tiger can cause serious problems.

The release of the Mac OS X 10.4.11 update was by no means a significant event, except for a few unlucky people who received a very nasty surprise after upgrading. Apparently there is a bug with the update, which will render some user's machines totally inoperable.
Although details are a bit sketchy, it looks like older Mac models, with Boot Camp partitions are affected. When updating via Software Update, users who get bitten by the bug will see an error message informing them that "Software Update has encountered an unexpected issue, you must restart."
Upon actually restarting, users will find that their Mac partitions hang on the blue loading screen, never progressing past this point, being unable to restart the machine in either OS X mode or Windows.

This bug threatens not only the data of the user, but also the loss of Boot Camp, which is no longer available for Tiger users to download and install. So far the number of affected systems appears to be quite small, but enough to see the pattern and that this could become a more widespread problem as people update their machines.
While discussions on the Apple support site have not been able to come up with any method of reversing the damage, or even determine the likelihood of the bug affecting any particular system, there are troubling reports that Apple's support staff suggests erasing the entire drive and a clean reinstall as the only way to resolve the problem.
Until a less destructive solution is found, or a way to single out systems that will be affected, it might be better if users err on the side of caution and skip this particular update.