Drop a Tab on Content as top element and you get the stacks below. What's more annoying is that you've to kill the SceneBuilder process, there's no possible recover (at least it's what I've seen on Mac).

Exception in thread "JavaFX Application Thread" java.lang.AssertionError
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.AbstractDecoration.startListeningToLayoutBounds(AbstractDecoration.java:177)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.driver.handles.TabHandles.startListeningToSceneGraphObject(TabHandles.java:102)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.AbstractDecoration$1.changed(AbstractDecoration.java:85)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.AbstractDecoration$1.changed(AbstractDecoration.java:79)
at com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(ExpressionHelper.java:176)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
at javafx.beans.property.ReadOnlyObjectWrapper$ReadOnlyPropertyImpl.fireValueChangedEvent(ReadOnlyObjectWrapper.java:176)
at javafx.beans.property.ReadOnlyObjectWrapper.fireValueChangedEvent(ReadOnlyObjectWrapper.java:142)
at javafx.scene.Node$ReadOnlyObjectWrapperManualFire.fireSuperValueChangedEvent(Node.java:802)
at javafx.scene.Node.invalidatedScenes(Node.java:851)
at javafx.scene.Node.setScenes(Node.java:860)
at javafx.scene.Parent$1.onChanged(Parent.java:276)
at com.sun.javafx.collections.TrackableObservableList$1.onChanged(TrackableObservableList.java:48)
at com.sun.javafx.collections.ListListenerHelper$Generic.fireValueChangedEvent(ListListenerHelper.java:315)
at com.sun.javafx.collections.ListListenerHelper.fireValueChangedEvent(ListListenerHelper.java:72)
at javafx.collections.ObservableListBase.fireChange(ObservableListBase.java:233)
at javafx.collections.ListChangeBuilder.commit(ListChangeBuilder.java:482)
at javafx.collections.ListChangeBuilder.endChange(ListChangeBuilder.java:541)
at javafx.collections.ObservableListBase.endChange(ObservableListBase.java:205)
at javafx.collections.ModifiableObservableListBase.add(ModifiableObservableListBase.java:155)
at java.util.AbstractList.add(AbstractList.java:108)
at com.sun.javafx.collections.VetoableListDecorator.add(VetoableListDecorator.java:200)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController.updateHandles(EditModeController.java:399)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController.updateHandles(EditModeController.java:341)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController.fxomDocumentDidRefreshSceneGraph(EditModeController.java:185)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.ContentPanelController.fxomDocumentDidRefreshSceneGraph(ContentPanelController.java:818)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.ContentPanelController.jobManagerRevisionDidChange(ContentPanelController.java:696)
at com.oracle.javafx.scenebuilder.kit.editor.panel.util.AbstractPanelController$3.changed(AbstractPanelController.java:187)
at com.oracle.javafx.scenebuilder.kit.editor.panel.util.AbstractPanelController$3.changed(AbstractPanelController.java:184)
at com.sun.javafx.binding.ExpressionHelper$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.JobManager.incrementRevision(JobManager.java:211)
at com.oracle.javafx.scenebuilder.kit.editor.JobManager.push(JobManager.java:76)
at com.oracle.javafx.scenebuilder.kit.editor.drag.DragController.end(DragController.java:107)
at com.oracle.javafx.scenebuilder.kit.editor.panel.library.LibraryPanelController$8.handle(LibraryPanelController.java:640)
at com.oracle.javafx.scenebuilder.kit.editor.panel.library.LibraryPanelController$8.handle(LibraryPanelController.java:635)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:204)
at javafx.scene.Scene$DnDGesture.processDropEnd(Scene.java:3017)
at javafx.scene.Scene$DnDGesture.access$6800(Scene.java:2867)
at javafx.scene.Scene$DragSourceListener.dragDropEnd(Scene.java:3252)
at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$6.run(GlassSceneDnDEventHandler.java:150)
at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$6.run(GlassSceneDnDEventHandler.java:145)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.handleDragEnd(GlassSceneDnDEventHandler.java:145)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleDragEnd(GlassViewEventHandler.java:615)
at com.sun.glass.ui.View.handleDragEnd(View.java:652)
at com.sun.glass.ui.View.notifyDragEnd(View.java:966)
at com.sun.glass.ui.mac.MacPasteboard._putItemsFromArray(Native Method)
at com.sun.glass.ui.mac.MacPasteboard.putItemsFromArray(MacPasteboard.java:143)
at com.sun.glass.ui.mac.MacPasteboard.putItems(MacPasteboard.java:165)
at com.sun.glass.ui.mac.MacSystemClipboard.pushToSystem(MacSystemClipboard.java:223)
at com.sun.glass.ui.SystemClipboard.flush(SystemClipboard.java:51)
at com.sun.glass.ui.ClipboardAssistance.flush(ClipboardAssistance.java:59)
at com.sun.javafx.tk.quantum.QuantumClipboard.flush(QuantumClipboard.java:274)
at com.sun.javafx.tk.quantum.QuantumToolkit.startDrag(QuantumToolkit.java:1149)
at javafx.scene.Scene$DnDGesture.dragDetectedProcessed(Scene.java:2911)
at javafx.scene.Scene$DnDGesture.process(Scene.java:2972)
at javafx.scene.Scene$DnDGesture.access$8400(Scene.java:2867)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3705)
at javafx.scene.Scene$MouseHandler.access$1800(Scene.java:3414)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1676)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2467)
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)
*** pgnodes.size validatePG() [0] != children.size() [1]
Exception in thread "JavaFX Application Thread" java.lang.AssertionError: validation of PGGroup children failed
at javafx.scene.Parent.validatePG(Parent.java:185)
at javafx.scene.Parent.impl_updatePeer(Parent.java:143)
at javafx.scene.Node.impl_syncPeer(Node.java:491)
at javafx.scene.Scene$ScenePulseListener.synchronizeSceneNodes(Scene.java:2264)
at javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:2386)
at com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:330)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:479)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:460)
at com.sun.javafx.tk.quantum.QuantumToolkit$13.run(QuantumToolkit.java:327)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at javafx.scene.Scene$ScenePulseListener.synchronizeSceneNodes(Scene.java:2263)
at javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:2386)
at com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:330)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:479)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:460)
at com.sun.javafx.tk.quantum.QuantumToolkit$13.run(QuantumToolkit.java:327)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)