Upgrading from X

[Aurelio Turco <azt@bom.gov.au>]

In the "Release Notes for Debian GNU/Linux 3.1"
it states:

  Distribution upgrade should be done either
  locally from a textmode virtual console
  (or a directly connected serial terminal),
  or remotely via an ssh link.

Whats the worse that can happen if a dist upgrade
is done from a local X session (started either
from xdm or startx)?

Cheers.
Aurelio.