Consider the stylesheet:

----- test.xsl
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

<xsl:template match="*[xsl:template][1]">
  <x/>
</xsl:template>

</xsl:stylesheet>
----- /test.xsl

Let run it:

----- screen
$ ~/p/third_party/libxslt/opt/bin/xsltproc test.xsl test.xsl
XPath error : Undefined namespace prefix
<?xml version="1.0"?>
<x/>
----- </screen>

Note the error "Undefined namespace prefix" is appeared, but there is no error. And, for example, Saxon doesn't complaint.

The funny thing is that if I change "*[xsl:template][1]" to "*[xsl:template]", the error is disappeared.