<ace:pushButton> doesn't support <f:ajax>. This makes debugging harder if you need to selectively disable Ajax behaviour to isolate issues.

Example:
           <ace:pushButton id="newOrderItem"
                    value="#{msg['com.edorasware.bpm.example.workbasket.new_order']}"
                    action="#{startNewOrderProcessAction.execute}">
                 <f:ajax disabled="true" />
            </ace:pushButton>

leads to this exception:
javax.faces.view.facelets.TagException: /WEB-INF/layout/defaultLoggedIn.xhtml @15,44 <f:ajax> Unable to attach <f:ajax> to non-ClientBehaviorHolder parent
com.sun.faces.facelets.tag.jsf.core.AjaxHandler.applyNested(AjaxHandler.java:297)
com.sun.faces.facelets.tag.jsf.core.AjaxHandler.apply(AjaxHandler.java:174)
javax.faces.view.facelets.DelegatingMetaTagHandler.applyNextHandler(DelegatingMetaTagHandler.java:137)
com.sun.faces.facelets.tag.jsf.ComponentTagHandlerDelegateImpl.apply(ComponentTagHandlerDelegateImpl.java:188)
javax.faces.view.facelets.DelegatingMetaTagHandler.apply(DelegatingMetaTagHandler.java:120)
javax.faces.view.facelets.CompositeFaceletHandler.apply(CompositeFaceletHandler.java:98)
javax.faces.view.facelets.DelegatingMetaTagHandler.applyNextHandler(DelegatingMetaTagHandler.java:137)
com.sun.faces.facelets.tag.jsf.ComponentTagHandlerDelegateImpl.apply(ComponentTagHandlerDelegateImpl.java:188)
javax.faces.view.facelets.DelegatingMetaTagHandler.apply(DelegatingMetaTagHandler.java:120)
com.sun.faces.facelets.tag.ui.DefineHandler.applyDefinition(DefineHandler.java:103)
com.sun.faces.facelets.tag.ui.CompositionHandler.apply(CompositionHandler.java:178)
com.sun.faces.facelets.impl.DefaultFaceletContext$TemplateManager.apply(DefaultFaceletContext.java:395)
com.sun.faces.facelets.impl.DefaultFaceletContext.includeDefinition(DefaultFaceletContext.java:366)
com.sun.faces.facelets.tag.ui.InsertHandler.apply(InsertHandler.java:108)
javax.faces.view.facelets.CompositeFaceletHandler.apply(CompositeFaceletHandler.java:98)
javax.faces.view.facelets.DelegatingMetaTagHandler.applyNextHandler(DelegatingMetaTagHandler.java:137)
com.sun.faces.facelets.tag.jsf.ComponentTagHandlerDelegateImpl.apply(ComponentTagHandlerDelegateImpl.java:188)
javax.faces.view.facelets.DelegatingMetaTagHandler.apply(DelegatingMetaTagHandler.java:120)
javax.faces.view.facelets.CompositeFaceletHandler.apply(CompositeFaceletHandler.java:98)
com.sun.faces.facelets.tag.jsf.core.ViewHandler.apply(ViewHandler.java:164)
javax.faces.view.facelets.CompositeFaceletHandler.apply(CompositeFaceletHandler.java:98)
com.sun.faces.facelets.compiler.NamespaceHandler.apply(NamespaceHandler.java:93)
javax.faces.view.facelets.CompositeFaceletHandler.apply(CompositeFaceletHandler.java:98)
com.sun.faces.facelets.compiler.EncodingHandler.apply(EncodingHandler.java:86)
com.sun.faces.facelets.impl.DefaultFacelet.include(DefaultFacelet.java:308)
com.sun.faces.facelets.impl.DefaultFacelet.include(DefaultFacelet.java:367)
com.sun.faces.facelets.impl.DefaultFacelet.include(DefaultFacelet.java:346)
com.sun.faces.facelets.impl.DefaultFaceletContext.includeFacelet(DefaultFaceletContext.java:199)
com.sun.faces.facelets.tag.ui.CompositionHandler.apply(CompositionHandler.java:155)
com.sun.faces.facelets.compiler.NamespaceHandler.apply(NamespaceHandler.java:93)
com.sun.faces.facelets.compiler.EncodingHandler.apply(EncodingHandler.java:86)
com.sun.faces.facelets.impl.DefaultFacelet.include(DefaultFacelet.java:308)
com.sun.faces.facelets.impl.DefaultFacelet.include(DefaultFacelet.java:367)
com.sun.faces.facelets.impl.DefaultFacelet.include(DefaultFacelet.java:346)
com.sun.faces.facelets.impl.DefaultFaceletContext.includeFacelet(DefaultFaceletContext.java:199)
com.sun.faces.facelets.tag.ui.CompositionHandler.apply(CompositionHandler.java:155)
com.sun.faces.facelets.compiler.NamespaceHandler.apply(NamespaceHandler.java:93)
com.sun.faces.facelets.compiler.EncodingHandler.apply(EncodingHandler.java:86)
com.sun.faces.facelets.impl.DefaultFacelet.apply(DefaultFacelet.java:152)
com.sun.faces.application.view.FaceletViewHandlingStrategy.buildView(FaceletViewHandlingStrategy.java:769)
com.sun.faces.lifecycle.RenderResponsePhase.execute(RenderResponsePhase.java:100)
com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
com.sun.faces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:139)
javax.faces.webapp.FacesServlet.service(FacesServlet.java:410)


This also happens with ICEfaces 2.0.1 Snapshot from 21 March 2011