Release:
xsltproc -V
Using libxml 20626, libxslt 10117 and libexslt 813
xsltproc was compiled against libxml 20626, libxslt 10117 and libexslt 813
libxslt 10117 was compiled against libxml 20626
libexslt 813 was compiled against libxml 20626

When a matching pattern like this is used, with "m" as prefix for the MathML namespace:

<xsl:template match="m:apply[1][@local='joke']">
  <xsl:message>Here</xsl:message>
  <xsl:apply-templates/>
</xsl:template>

xsltproc complains:
XPath error : Undefined namespace prefix
xmlXPathEval: evaluation failed

It seems due to the use of multiples expressions "[...]" because doing this works:

<xsl:template match="m:apply[position()=1 and @local='joke']">

It also fails for things like match="m:apply[1][m:abs]"

A complete example:

The XSL stylesheet
------------------------
<?xml version='1.0'?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:m="http://www.w3.org/1998/Math/MathML"
                version='1.0'>

<xsl:output method="text" encoding="ISO-8859-1" indent="yes"/>

<xsl:template match="inlineequation|informalequation">
  <xsl:apply-templates/>
</xsl:template>

<xsl:template match="equation">
  <xsl:message>OK</xsl:message>
  <xsl:apply-templates/>
</xsl:template>

<xsl:template match="/">
  <xsl:apply-templates/>
</xsl:template>

<xsl:template match="m:math">
  <xsl:apply-templates/>
</xsl:template>

<xsl:template match="m:apply[1][@local='joke']">
  <xsl:message>Here</xsl:message>
  <xsl:apply-templates/>
</xsl:template>

</xsl:stylesheet>
----------------------

The XML doc:
----------------------
<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook MathML Module V1.0//EN"
"http://www.oasis-open.org/docbook/xml/mathml/1.0/dbmathml.dtd">

<article>
  <title>Titre</title>
  <section>
    <title>Introduction</title>
    <para> test </para>
    <equation>
      <title>A title</title>
      <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
        <mml:apply>
          <mml:abs/>
          <mml:apply>
            <mml:divide/>
            <mml:ci> H </mml:ci>
            <mml:ci> K </mml:ci>
          </mml:apply>
        </mml:apply>
      </mml:math>
    </equation>
  </section>
</article>
-------------------