I got this at least once while debugging Ensemble8 media samples.

The app hung upon closing with this trace:

"JavaFX Application Thread" #12 prio=5 os_prio=0 tid=0x0000000018dc4800 nid=0x1260 runnable [0x0000000019d4e000]
   java.lang.Thread.State: RUNNABLE
at com.sun.media.jfxmediaimpl.platform.gstreamer.GSTMedia.gstDispose(Native Method)
at com.sun.media.jfxmediaimpl.platform.gstreamer.GSTMedia.dispose(GSTMedia.java:83)
- locked <0x00000000897a9540> (a com.sun.media.jfxmediaimpl.platform.gstreamer.GSTMedia)
at com.sun.media.jfxmediaimpl.NativeMediaPlayer.dispose(NativeMediaPlayer.java:1291)
at javafx.scene.media.MediaPlayerShutdownHook.run(MediaPlayer.java:2853)
- locked <0x00000000864558e8> (a java.util.ArrayList)
at com.sun.javafx.tk.Toolkit.notifyShutdownHooks(Toolkit.java:402)
at com.sun.javafx.tk.quantum.QuantumToolkit.exit(QuantumToolkit.java:680)
at com.sun.javafx.application.PlatformImpl$10.run(PlatformImpl.java:461)
at com.sun.javafx.application.PlatformImpl.runAndWait(PlatformImpl.java:321)
at com.sun.javafx.application.PlatformImpl.tkExit(PlatformImpl.java:458)
at com.sun.javafx.application.PlatformImpl.notifyFinishListeners(PlatformImpl.java:383)
at com.sun.javafx.application.PlatformImpl.checkIdle(PlatformImpl.java:440)
at com.sun.javafx.application.PlatformImpl.access$800(PlatformImpl.java:51)
at com.sun.javafx.application.PlatformImpl$6.run(PlatformImpl.java:303)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$300(WinApplication.java:39)
at com.sun.glass.ui.win.WinApplication$4$1.run(WinApplication.java:112)
at java.lang.Thread.run(Thread.java:744)