Hi Michael, could you take a look at this bug report?  

If you have problems with the attachments, the bug can be viewed online
at http://emacsbugs.donarmstrong.com/cgi-bin/bugreport.cgi?bug=1742

Thanks.

>>> David Abrahams <dave@boostpro.com> wrote:

0. Extract the enclosed files to /tmp

1. Evaluate:

  (custom-set-variables
   '(ediff-skip-merge-regions-that-differ-from-default t)
  )
  (ediff "/tmp/x" "/tmp/y")
  
2. Hit the spacebar

Result: "You have killed a vital Ediff buffer---you must leave Ediff
now!"

This happens even with "emacs -q"