To reproduce, create a TitlePane, dynamic set mnemonicParsing property, IndexOutOfBoundsException is reported when setting it to false.
sample program attached.

java.lang.IndexOutOfBoundsException: Index: 2, Size: 2
at java.util.ArrayList.rangeCheck(ArrayList.java:604)
at java.util.ArrayList.get(ArrayList.java:382)
at com.sun.javafx.collections.ObservableListWrapper.get(ObservableListWrapper.java:63)
at com.sun.javafx.collections.VetoableListDecorator.get(VetoableListDecorator.java:246)
at javafx.scene.Parent.layout(Parent.java:1018)
at javafx.scene.Parent.layout(Parent.java:1020)
at javafx.scene.Scene.layoutDirtyRoots(Scene.java:521)
at javafx.scene.Scene.doLayoutPass(Scene.java:492)
at javafx.scene.Scene.access$3900(Scene.java:176)
at javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:2232)
at com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:345)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:526)
at com.sun.javafx.tk.quantum.QuantumToolkit$12.run(QuantumToolkit.java:379)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(Unknown Source)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$300(Unknown Source)
at com.sun.glass.ui.win.WinApplication$3$1.run(Unknown Source)
at java.lang.Thread.run(Thread.java:722)