Using nested custom tags that create scripting variables leads Jasper to create invalid code. Example:

<my:bean name="outer">
        <%-- outer is null iff pageContext.findAttribute("outer") is null --%>
        <my:bean name="inner"/>
        <%-- inner is always null even if pageContext.findAttribute("inner") is not  --%>
 </my:bean>
<%-- outer is null iff pageContext.findAttribute("outer") is null --%>

where the bean-tag sets a value in the pageContext with the given name and any value.
This bug is known since tomcat version 4.1 (see bug 19361).