IE 6 crashes after you install security update MS07-069 on a computer that is running Windows XP SP2
This happened on one of my PCs last night - you need to edit the registry to fix it, but Jesper Johansson's got an installer that will do that for you.
Update:
http://support.microsoft.com/kb/946627 has been updated.
It now has a description of the IE6 crash and a link to a file you can download from the Microsoft Download site to add the registry entry required to prevent the crash.
I also noticed on one of my PCs at home this morning that Microsoft Update is pushing this out as a high-priority update.