There was a problem when installing Snow Leopard, where your computer would crash on boot if you had a specific third party extension installed (I think it was

There was a problem when installing Snow Leopard, where your computer would crash on boot if you had a specific third party extension installed (I think it was Application Enhancer). I assume to prevent similar problems, Apple started disabling incompatible software after system upgrades in Lion. The blacklist can't really be called "quiet": On first boot after system upgrade, a dialog box pops up that tells you that incompatible software was moved to a special folder. The affected stuff is mostly kernel extensions that become incompatible.