According to w3 org if the namespace prefix specified in 'exclude-result-prefixes' and it is not defined in the xsl file - it is an error:
"[ERR XTSE0808] It is a static error if a namespace prefix is used within the [xsl:]exclude-result-prefixes attribute and there is no namespace binding in scope for that prefix."

The following XSL transformation should fail with exception, but it finishes without errors:
<xsl:stylesheet version='2.0'"
 xmlns:xsl='http://www.w3.org/1999/XSL/Transform&#39;
 xmlns:good_prefix='this_is_good_prefix'
 exclude-result-prefixes='good_prefix bad_prefix'>
 <xsl:output method='xml' indent='yes' omit-xml-declaration='yes'/>
 <xsl:template match='/'>
 <test>You shouldn't see it</test>
 </xsl:template>
</xsl:stylesheet>