The following transform provides a minimal test case for a problem that arose in the XSLT-based C code generator for the XML-XCB project.  This transform performs multi-pass processing using the EXSLT node-set extension.  The bug arose when I attempted to define a variable in the stylesheet containing some fixed data and traverse it as part of the definition of one of the variables used for multi-pass processing; both variables consist of a result-tree-fragment subsequently passed to the node-set extension.  If you run this transform, the first access to the request-suffixes variable via xsl:for-each successfully iterates over the two elements it contains, but the second access sees no content in the variable.

test.xsl:
<?xml version="1.0" encoding="utf-8"?>
<xsl:transform xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
               version="1.0"
               xmlns:e="http://exslt.org/common"
               exclude-result-prefixes="e">
  <xsl:variable name="request-suffixes-rtf">
    <checked />
    <checked />
  </xsl:variable>
  <xsl:variable name="request-suffixes"
                select="e:node-set($request-suffixes-rtf)" />

  <xsl:template match="request" mode="pass1">
    <thing1>
    <xsl:for-each select="$request-suffixes/checked">
      <thing2 />
    </xsl:for-each>
    </thing1>
  </xsl:template>

  <xsl:variable name="pass1-rtf">
    <xsl:apply-templates select="/" mode="pass1" />
  </xsl:variable>
  <xsl:variable name="pass1" select="e:node-set($pass1-rtf)" />

  <xsl:template match="/">
    <root>
      <xsl:copy-of select="$pass1/*" />
    </root>
  </xsl:template>
</xsl:transform>

test.xml:
<?xml version="1.0" encoding="utf-8"?>
<xcb>
  <request />
  <request />
</xcb>

Run this example with: xsltproc test.xsl test.xml | xmllint --format -

Expected result:
<?xml version="1.0"?>
<root>
  <thing1>
    <thing2/>
    <thing2/>
  </thing1>
  <thing1>
    <thing2/>
    <thing2/>
  </thing1>
</root>

Actual result:
<?xml version="1.0"?>
<root>
  <thing1>
    <thing2/>
    <thing2/>
  </thing1>
  <thing1/>
</root>

Excerpts from the xsltproc -v debugging output:
[...]
xsltForEach: select $request-suffixes/checked
Lookup variable request-suffixes
Evaluating global variable request-suffixes
Lookup variable request-suffixes-rtf
Evaluating global variable request-suffixes-rtf
reusing transformation dict for RVT
xsltApplyOneTemplate: copy node checked
xsltApplyOneTemplate: copy node checked
Object is an XSLT value tree :
1  ELEMENT checked
  ELEMENT checked
found variable request-suffixes-rtf
Lookup function {http://exslt.org/common}node-set
found function node-set
Object is a Node Set :
Set contains 1 nodes:
1   /
found variable request-suffixes
xsltForEach: select evaluates to 2 nodes
xsltForEach: Changing document - context doc temp.xml, xpathdoc (null)
xsltApplyOneTemplate: copy node thing2
xsltApplyOneTemplate: copy node thing2
[...]
xsltForEach: select $request-suffixes/checked
Lookup variable request-suffixes
found variable request-suffixes
xsltForEach: select evaluates to 0 nodes