1) In the fourth example in section 9.8 of XSLT 2.0,[1] entitled "Example: Circularity involving Variables and Keys," the following stylesheet function appears:

<xsl:function name="my:f">
  <xsl:param name="arg1"/>
  <xsl:sequence select="key('k', $arg1)"/>
</xsl:function>

If a reference to the key function has only two arguments, the reference returns nodes in the same document as the context node.  However, the context item is initially undefined in a stylesheet function, so it's implementation dependent whether this example will result in an error because of the circularity or because the context item is undefined.

The example could either be modified to eliminate the stylesheet function, or a second argument to the stylesheet function could be added to use as the $top argument in the reference to the key function.

2) In the final example of section 9.8, there is a quotation mark missing in the xsl:variable declaration:

<xsl:variable name="x" select="my:f(1)/>

should be

<xsl:variable name="x" select="my:f(1)"/>

[1] http://www.w3.org/TR/xslt20/#circularity