Using document('doc.xml')/* where doc.xml doesn't exist produces a 
crash.  It is only when you add additional logic behind hte document() 
function, i.e. the following works

<xsl:copy-of select="document('doc.xml')"/>

and the following crashes

<xsl:copy-of select="document('doc.xml')/*/*"/>

when doc.xml doesn't exist.

(note:  gssxslt is xsltproc with two additional lines to register custom 
extensions)

C:\>gssxslt --version
Using libxml 20501, libxslt 10024 and libexslt 715
xsltproc was compiled against libxml 20501, libxslt 10024 and libexslt 715
libxslt 10024 was compiled against libxml 20501
libexslt 715 was compiled against libxml 20501

See attached sample stylesheet