This segfaults me on GNU Emacs 24.0.50.1 (i686-pc-linux-gnu, GTK+ Version
2.16.1) of 2010-05-09:

  (all-completions "" [])

I think the problem is on line 1593 of src/minibuf.c, but my
Emacs-innards-fu isn't good enough to attempt a fix.