Because tomcat 4.1.12 use a tag pool, when you use style tag using xml or xsl 
inside body, it throw an exception once on two.
I know why and I know how to correct it :

    public int doAfterBody() throws JspException {
        if ( bodyContent != null ) {
            String text = bodyContent.getString().trim();
            if ( text.length() > 0 ) {
                if ( xsl == null ) {^
                   ^^^^^^^^^^^^^^^^^^
                    xsl = new StringReader( text );
                }
                else if ( xml == null ) {
                    ^^^^^^^^^^^^^^^^^^
                    xml = new StringReader( text );
                }
...
It test if xml or xsl is null, or because it use a pool, first time it access 
to xml in body, it create a StringReader, secund times, it did not create a 
new one (because xml is not null) and you will have an exception because this 
reader is closed.

To correct this bug, add destroying xml and xsl variable in doEndTag in 
finally section :

        finally {
            stringWriter = null;
            xml=null;
            xsl=null;
        }

Best,
Teva Lautier