The exception I get when XEP is added to the stack is this:
HTTP Status 500 -
type Exception report
message
description The server encountered an internal error () that prevented it from fulfilling this request.
exception
javax.servlet.ServletException: Servlet execution threw an exception

root cause
java.lang.UnsupportedClassVersionError: Bad version number in .class file
java.lang.ClassLoader.defineClass1(Native Method)
java.lang.ClassLoader.defineClass(ClassLoader.java:620)
java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
org.apache.catalina.loader.WebappClassLoader.findClassInternal(WebappClassLoader.java:1815)
org.apache.catalina.loader.WebappClassLoader.findClass(WebappClassLoader.java:872)
org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1325)
org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1204)
org.apache.cocoon.components.ExtendedComponentSelector.configure(ExtendedComponentSelector.java:227)
org.apache.cocoon.components.treeprocessor.sitemap.ComponentsSelector.configure(ComponentsSelector.java:157)
org.apache.avalon.framework.container.ContainerUtil.configure(ContainerUtil.java:201)
org.apache.avalon.excalibur.component.DefaultComponentFactory.newInstance(DefaultComponentFactory.java:289)
org.apache.avalon.excalibur.component.ThreadSafeComponentHandler.initialize(ThreadSafeComponentHandler.java:108)
org.apache.avalon.excalibur.component.ExcaliburComponentManager.initialize(ExcaliburComponentManager.java:524)
org.apache.cocoon.components.CocoonComponentManager.initialize(CocoonComponentManager.java:566)
org.apache.cocoon.components.treeprocessor.sitemap.SitemapLanguage.createComponentManager(SitemapLanguage.java:90)
org.apache.cocoon.components.treeprocessor.DefaultTreeBuilder.build(DefaultTreeBuilder.java:389)
org.apache.cocoon.components.treeprocessor.DefaultTreeBuilder.build(DefaultTreeBuilder.java:369)
org.apache.cocoon.components.treeprocessor.TreeProcessor.buildConcreteProcessor(TreeProcessor.java:339)
org.apache.cocoon.components.treeprocessor.TreeProcessor.setupConcreteProcessor(TreeProcessor.java:304)
org.apache.cocoon.components.treeprocessor.TreeProcessor.process(TreeProcessor.java:250)
org.apache.cocoon.Cocoon.process(Cocoon.java:686)
org.apache.cocoon.servlet.CocoonServlet.service(CocoonServlet.java:1153)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)

note The full stack trace of the root cause is available in the Apache Tomcat/6.0.7 logs.

This is a result of building XEP jars against a newer JVM than the version running the servlet. In this case, I'm building the jars under 1.6 and running the Tomcat stack under 1.5 on lettuce. Rather than rebuilding XEP, I'm going to get 1.6 installed on lettuce, as it's our "latest-and-greatest". Also, 1.6 is the current stable JVM from Sun.