After using Ensemble for a little bit, moving mouse over the icons for demos throws:

java.lang.ClassCastException: com.sun.javafx.geom.BoxBounds cannot be cast to com.sun.javafx.geom.RectBounds
        at com.sun.javafx.sg.BaseNode.accumulateGroupDirtyRegion(BaseNode.java:817)
        at com.sun.javafx.sg.BaseNode.accumulateDirtyRegions(BaseNode.java:694)
        at com.sun.javafx.sg.BaseNode.accumulateGroupDirtyRegion(BaseNode.java:852)
        at com.sun.javafx.sg.BaseNode.accumulateDirtyRegions(BaseNode.java:694)
        at com.sun.javafx.sg.BaseNode.accumulateGroupDirtyRegion(BaseNode.java:852)
        at com.sun.javafx.sg.BaseNode.accumulateDirtyRegions(BaseNode.java:694)
        at com.sun.javafx.sg.BaseNode.accumulateGroupDirtyRegion(BaseNode.java:852)
        at com.sun.javafx.sg.BaseNode.accumulateDirtyRegions(BaseNode.java:694)
        at com.sun.javafx.sg.BaseNode.accumulateGroupDirtyRegion(BaseNode.java:852)
        at com.sun.javafx.sg.BaseNode.accumulateDirtyRegions(BaseNode.java:694)
        at com.sun.javafx.sg.BaseNode.accumulateGroupDirtyRegion(BaseNode.java:852)
        at com.sun.javafx.sg.BaseNode.accumulateDirtyRegions(BaseNode.java:694)
        at com.sun.javafx.sg.BaseNode.accumulateGroupDirtyRegion(BaseNode.java:852)
        at com.sun.javafx.sg.BaseNode.accumulateDirtyRegions(BaseNode.java:694)
        at com.sun.javafx.sg.BaseNode.accumulateGroupDirtyRegion(BaseNode.java:852)
        at com.sun.javafx.sg.BaseNode.accumulateDirtyRegions(BaseNode.java:694)
        at com.sun.javafx.sg.BaseNode.accumulateGroupDirtyRegion(BaseNode.java:852)
        at com.sun.javafx.sg.BaseNode.accumulateDirtyRegions(BaseNode.java:694)
        at com.sun.javafx.sg.BaseNode.accumulateGroupDirtyRegion(BaseNode.java:852)
        at com.sun.javafx.sg.BaseNode.accumulateDirtyRegions(BaseNode.java:694)
        at com.sun.javafx.tk.quantum.PaintRunnable.setDirtyRect(PaintRunnable.java:98)
        at com.sun.javafx.tk.quantum.PaintRunnable.paintImpl(PaintRunnable.java:118)
        at com.sun.javafx.tk.quantum.PaintRunnable.run(PaintRunnable.java:236)
        at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
        at java.util.concurrent.FutureTask$Sync.innerRunAndReset(Unknown Source)
        at java.util.concurrent.FutureTask.runAndReset(Unknown Source)
        at com.sun.prism.render.RenderJob.run(RenderJob.java:29)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at com.sun.javafx.tk.quantum.QuantumRenderer$ObservedRunnable.run(QuantumRenderer.java:70)
        at java.lang.Thread.run(Unknown Source)


Steps:
It can be somewhat tricky to reproduce 100% of the time but it will happen
Launch Ensemble
Using the keyboard traverse down the demo tree until you come to the Shapes node
Mouse over the different demo icons. Now every mouse move anywhere throws the exception over and over