src/main/java/org/efaps/ui/wicket/behaviors/AjaxFieldUpdateBehavior.java
@Override
    protected void onError(final AjaxRequestTarget _target)
    {
        if (this.errorHandling) {
            final FeedbackCollector collector = new FeedbackCollector(getForm().getPage());
            final List<FeedbackMessage> msgs = collector.collect();
            final StringBuilder html = new StringBuilder()
                            .append("<table class=\"eFapsValidateFieldValuesTable\">");
            for (final FeedbackMessage msg : msgs) {
                msg.getReporter().add(AttributeModifier.append("class", "invalid"));
                _target.add(msg.getReporter());
                Serializable warn = null;
                if (msg.getMessage() instanceof ValidationErrorFeedback) {
                    // look if a message was set
                    warn = ((ValidationErrorFeedback) msg.getMessage()).getMessage();
                    // still no message, create one
                    if (warn == null) {
                        warn = ((ValidationErrorFeedback) msg.getMessage()).getError().getErrorMessage(
                                        new ErrorMessageResource());
                    }
                }
                String label = "";
                if (msg.getReporter() instanceof IFieldConfig) {
                    label = ((IFieldConfig) msg.getReporter()).getFieldConfig().getLabel();
                }
                html.append("<tr><td>").append(label).append(":</td><td>")
                                .append(warn).append("</td></tr>");
            }
            html.append("</table>");

            final ModalWindowContainer modal = ((AbstractContentPage) getComponent().getPage()).getModal();

            modal.setInitialWidth(350);
            modal.setInitialHeight(200);

            modal.setPageCreator(new ModalWindow.PageCreator()
            {

                private static final long serialVersionUID = 1L;

                @Override
                public Page createPage()
                {
                    return new DialogPage(((AbstractContentPage) getComponent().getPage()).getPageReference(),
                                    html.toString(), true, false);
                }
            });
            modal.show(_target);
        }
    }
@Override
    protected void onError(final AjaxRequestTarget _target)
    {
        if (this.errorHandling) {
            final FeedbackCollector collector = new FeedbackCollector(getForm().getPage());
            final List<FeedbackMessage> msgs = collector.collect();
            final StringBuilder html = new StringBuilder()
                            .append("<table class=\"eFapsValidateFieldValuesTable\">");
            for (final FeedbackMessage msg : msgs) {
                msg.getReporter().add(AttributeModifier.append("class", "invalid"));
                _target.add(msg.getReporter());
                Serializable warn = null;
                if (msg.getMessage() instanceof ValidationErrorFeedback) {
                    // look if a message was set
                    warn = ((ValidationErrorFeedback) msg.getMessage()).getMessage();
                    // still no message, create one
                    if (warn == null) {
                        warn = ((ValidationErrorFeedback) msg.getMessage()).getError().getErrorMessage(
                                        new ErrorMessageResource());
                    }
                }
                String label = "";
                if (msg.getReporter() instanceof IFieldConfig) {
                    label = ((IFieldConfig) msg.getReporter()).getFieldConfig().getLabel();
                }
                html.append("<tr><td>").append(label).append(":</td><td>")
                                .append(warn).append("</td></tr>");
            }
            html.append("</table>");

            final ModalWindowContainer modal = ((AbstractContentPage) getComponent().getPage()).getModal();

            modal.setInitialWidth(350);
            modal.setInitialHeight(200);

            modal.setPageCreator(new ModalWindow.PageCreator()
            {

                private static final long serialVersionUID = 1L;

                @Override
                public Page createPage()
                {
                    return new DialogPage(((AbstractContentPage) getComponent().getPage()).getPageReference(),
                                    html.toString(), true, false);
                }
            });
            modal.show(_target);
        }
    }
Default means nothing is done on error.

@see org.apache.wicket.ajax.form.AjaxFormSubmitBehavior#onError(org.apache.wicket.ajax.AjaxRequestTarget)
@param _target AjaxRequestTarget