Comment 4 for bug 439551

mvo recently added some code to the installer to remove xorg.conf if it is 0-bytes, so this situation should not crop up as frequently in ordinary use.

I don't seem to have permission to view bug 457433, but will assume it sufficiently covers the virtualbox failure on upgrade, that we can consider this report closed. Note that virtualbox provides its own binary X drivers, compiled against a specific X version so if you upgrade xserver to a new major version without also upgrading virtualbox, I could imagine it would cause X crashes - but that's going to be a virtualbox bug rather than Xorg, since virtualbox just probably needs its drivers to depend on Xorg versions properly or something.