Appeared here :
http://stackoverflow.com/questions/16375967/javafx-2-setting-the-defaultbutton-property-mutually-exclusive

Exception :

java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:894)
at java.util.HashMap$EntryIterator.next(HashMap.java:934)
at java.util.HashMap$EntryIterator.next(HashMap.java:932)
at com.sun.javafx.collections.ObservableMapWrapper$ObservableEntrySet$1.next(ObservableMapWrapper.java:560)
at com.sun.javafx.collections.ObservableMapWrapper$ObservableEntrySet$1.next(ObservableMapWrapper.java:548)
at com.sun.javafx.scene.KeyboardShortcutsHandler.processAccelerators(KeyboardShortcutsHandler.java:286)
at com.sun.javafx.scene.KeyboardShortcutsHandler.dispatchBubblingEvent(KeyboardShortcutsHandler.java:119)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:28)
at javafx.event.Event.fireEvent(Event.java:171)
at javafx.scene.Scene$KeyHandler.process(Scene.java:3513)
at javafx.scene.Scene$KeyHandler.access$2300(Scene.java:3472)
at javafx.scene.Scene.impl_processKeyEvent(Scene.java:1904)
at javafx.scene.Scene$ScenePeerListener.keyEvent(Scene.java:2270)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:136)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:100)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleKeyEvent(GlassViewEventHandler.java:163)
at com.sun.glass.ui.View.handleKeyEvent(View.java:518)
at com.sun.glass.ui.View.notifyKey(View.java:951)
at com.sun.glass.ui.win.WinApplication._enterNestedEventLoop(Native Method)
at com.sun.glass.ui.Application.enterNestedEventLoop(Application.java:383)
at com.sun.glass.ui.EventLoop.enter(EventLoop.java:83)
at com.sun.javafx.tk.quantum.QuantumToolkit.enterNestedEventLoop(QuantumToolkit.java:520)
at javafx.stage.Stage.showAndWait(Stage.java:397)
at org.kls.md.censusassistant.DialogController.showAndWait(DialogController.java:94)
at org.kls.md.censusassistant.DCMainEditor.handleEncDetails(DCMainEditor.java:287)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:75)
at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:279)
at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1435)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:28)
at javafx.event.Event.fireEvent(Event.java:171)
at javafx.scene.Node.fireEvent(Node.java:6863)
at javafx.scene.control.Button.fire(Button.java:179)
at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:193)
at com.sun.javafx.scene.control.skin.SkinBase$4.handle(SkinBase.java:336)
at com.sun.javafx.scene.control.skin.SkinBase$4.handle(SkinBase.java:329)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:64)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)
at javafx.event.Event.fireEvent(Event.java:171)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3328)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3168)
at javafx.scene.Scene$MouseHandler.access$1900(Scene.java:3123)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1563)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2265)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:250)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:173)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:292)
at com.sun.glass.ui.View.handleMouseEvent(View.java:528)
at com.sun.glass.ui.View.notifyMouse(View.java:922)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$100(WinApplication.java:29)
at com.sun.glass.ui.win.WinApplication$3$1.run(WinApplication.java:73)
at java.lang.Thread.run(Thread.java:722)

Seems, the trouble is in this code :

private void setDefaultButton(boolean value) {

    KeyCode acceleratorCode = KeyCode.ENTER;
    defaultAcceleratorKeyCodeCombination =
            new KeyCodeCombination(acceleratorCode);

    if (! value) {
        /*
        ** first check of there's a default button already
        */
        Runnable oldDefault = getSkinnable().getParent().getScene().getAccelerators().get(defaultAcceleratorKeyCodeCombination);
        if (!defaultButtonRunnable.equals(oldDefault)) {
            /*
            ** is it us?
            */
            getSkinnable().getParent().getScene().getAccelerators().remove(defaultAcceleratorKeyCodeCombination);
        }
    }
    getSkinnable().getParent().getScene().getAccelerators().put(defaultAcceleratorKeyCodeCombination, defaultButtonRunnable);
}

Where remove is used, which causes the concurrent modification exception. I tried to create an application, but couldn't deal with logic of event processing order to recreate the issue.