In class org.apache.xpath.axes.AxesWalker method getLastPos there's a fragment
of code:
 walker.setPredicateCount(walker.getPredicateCount() - 1);

Thus, in expression x[last() != position()][true()] first predicate is checked
twice.

XML sample:
<?xml version="1.0" encoding="UTF-8"?>
<doc>
   <item>1</item>
   <item>2</item>
   <item>3</item>
</doc>

Result of
 <xsl:copy-of select="/doc/item[last() != position()][true()]"/>

is
 <item>1</item>

and should be
 <item>1</item>
 <item>2</item>

XSL work-around:
 <xsl:copy-of select="(/doc/item[last() != position()])[true()]"/>

Java code fix: replace code line in org.apache.xpath.axes.AxesWalker getLastPos:
 walker.setPredicateCount(walker.getPredicateCount() - 1);
to
 walker.setPredicateCount(this.m_predicateIndex);