If one sets stacked="true" to an ace:column component that is a child of ace:row and ace:columnGroup, there's an exception thrown in the console and also displayed on the page.

The markup looks like this:

                <ace:columnGroup type="header">
                    <ace:row>
                        <ace:column >
                            <f:facet name="header">
                                Item One
                            </f:facet>
                        </ace:column>
                        <ace:column stacked="true">
                            <f:facet name="header">
                                Item Two
                            </f:facet>
                        </ace:column>
                        ...

...and this is the exception thrown:

java.lang.RuntimeException: failed to append text[] into #document
        at org.icefaces.impl.context.DOMResponseWriter.appendToCursor(DOMResponseWriter.java:406)
        at org.icefaces.impl.context.DOMResponseWriter.appendToCursor(DOMResponseWriter.java:382)
        at org.icefaces.impl.context.DOMResponseWriter.writeText(DOMResponseWriter.java:323)
        at com.sun.faces.facelets.compiler.LiteralTextInstruction.write(LiteralTextInstruction.java:76)
        at com.sun.faces.facelets.compiler.UIInstructions.encodeBegin(UIInstructions.java:82)
        at com.sun.faces.facelets.compiler.UILeaf.encodeAll(UILeaf.java:183)
        at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1759)
        at com.sun.faces.application.view.FaceletViewHandlingStrategy.renderView(FaceletViewHandlingStrategy.java:401)
        at com.sun.faces.application.view.MultiViewHandler.renderView(MultiViewHandler.java:131)
        at com.sun.faces.lifecycle.RenderResponsePhase.execute(RenderResponsePhase.java:121)
        at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
        at com.sun.faces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:139)
        at javax.faces.webapp.FacesServlet.service(FacesServlet.java:594)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:240)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:164)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:462)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
        at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:563)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:399)
        at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:317)
        at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:204)
        at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:182)
        at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:311)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
        at java.lang.Thread.run(Thread.java:662)
Caused by: org.w3c.dom.DOMException: HIERARCHY_REQUEST_ERR: An attempt was madeto insert a node where it is not permitted.
        at com.sun.org.apache.xerces.internal.dom.ParentNode.internalInsertBefore(ParentNode.java:356)
        at com.sun.org.apache.xerces.internal.dom.ParentNode.insertBefore(ParentNode.java:284)
        at com.sun.org.apache.xerces.internal.dom.CoreDocumentImpl.insertBefore(oreDocumentImpl.java:399)
        at com.sun.org.apache.xerces.internal.dom.NodeImpl.appendChild(NodeImpl.java:235)
        at org.icefaces.impl.context.DOMResponseWriter.appendToCursor(DOMResponseWriter.java:401)
        ... 29 more