Start the tool
Show CSS analyzer
DND an AnchorPane, then 2 buttons in the AP.
Try to select the 2 buttons.
You wil get the following assertion error :

Exception in thread "JavaFX Application Thread" java.lang.AssertionError
at com.oracle.javafx.scenebuilder.kit.editor.panel.css.CssPanelController.getFXOMInstance(CssPanelController.java:1474)
at com.oracle.javafx.scenebuilder.kit.editor.panel.css.CssPanelController.fillSelectionContent(CssPanelController.java:638)
at com.oracle.javafx.scenebuilder.kit.editor.panel.css.CssPanelController.setCSSContent(CssPanelController.java:605)
at com.oracle.javafx.scenebuilder.kit.editor.panel.css.CssPanelController.refresh(CssPanelController.java:568)
at com.oracle.javafx.scenebuilder.kit.editor.panel.css.CssPanelController.editorSelectionDidChange(CssPanelController.java:240)
at com.oracle.javafx.scenebuilder.kit.editor.panel.util.AbstractPanelController$5.changed(AbstractPanelController.java:213)
at com.oracle.javafx.scenebuilder.kit.editor.panel.util.AbstractPanelController$5.changed(AbstractPanelController.java:210)
at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(ExpressionHelper.java:347)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
at javafx.beans.property.IntegerPropertyBase.fireValueChangedEvent(IntegerPropertyBase.java:106)
at javafx.beans.property.IntegerPropertyBase.markInvalid(IntegerPropertyBase.java:113)
at javafx.beans.property.IntegerPropertyBase.set(IntegerPropertyBase.java:146)
at com.oracle.javafx.scenebuilder.kit.editor.selection.Selection.incrementRevision(Selection.java:487)
at com.oracle.javafx.scenebuilder.kit.editor.selection.Selection.endUpdate(Selection.java:455)
at com.oracle.javafx.scenebuilder.kit.editor.selection.Selection.select(Selection.java:371)
at com.oracle.javafx.scenebuilder.kit.editor.selection.Selection.toggleSelection(Selection.java:193)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.gesture.mouse.SelectAndMoveGesture.mousePressed(SelectAndMoveGesture.java:126)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.gesture.mouse.AbstractMouseDragGesture.start(AbstractMouseDragGesture.java:103)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController.activateGesture(EditModeController.java:872)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController.mousePressedOnGlassLayer(EditModeController.java:704)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController.access$300(EditModeController.java:93)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController$4.handle(EditModeController.java:514)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController$4.handle(EditModeController.java:511)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:204)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3746)
at javafx.scene.Scene$MouseHandler.access$1800(Scene.java:3471)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1695)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2486)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:314)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:243)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:345)
at com.sun.glass.ui.View.handleMouseEvent(View.java:526)
at com.sun.glass.ui.View.notifyMouse(View.java:898)