Seems to be a regression possibly introduced with 8u72. The exception is thrown on focus lost and if type is not String and a valueProperty listener is installed just like in the example below. The issue does not occur with 8u73.

import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.geometry.Pos;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.ComboBox;
import javafx.stage.Stage;

public class ComboBoxCCE extends Application {

    private Parent getContent() {
        ComboBox<Pos> combo = new ComboBox<>();
        combo.setItems(FXCollections.observableArrayList(Pos.TOP_LEFT, Pos.TOP_CENTER, Pos.TOP_RIGHT, Pos.CENTER_LEFT, Pos.CENTER, Pos.CENTER_RIGHT, Pos.BOTTOM_LEFT, Pos.BOTTOM_CENTER, Pos.BOTTOM_RIGHT));
        combo.setValue(null);
        combo.valueProperty().addListener((o, ov, nv) -> {
            System.err.println("--- New Value: " + nv);
        });
        combo.setEditable(true);
        return combo;
    }
     
    @Override
    public void start(Stage primaryStage) throws Exception {
        primaryStage.setScene(new Scene(getContent()));
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}

Exception in thread "JavaFX Application Thread" java.lang.ClassCastException: java.lang.String cannot be cast to javafx.geometry.Pos
        at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(Unknown Source)
        at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(Unknown Source)
        at javafx.beans.property.ObjectPropertyBase.fireValueChangedEvent(Unknown Source)
        at javafx.beans.property.ObjectPropertyBase.markInvalid(Unknown Source)
        at javafx.beans.property.ObjectPropertyBase.set(Unknown Source)
        at javafx.scene.control.ComboBoxBase.setValue(Unknown Source)
        at com.sun.javafx.scene.control.skin.ComboBoxPopupControl.setTextFromTextFieldIntoComboBoxValue(Unknown Source)
        at com.sun.javafx.scene.control.skin.ComboBoxPopupControl.lambda$new$291(Unknown Source)
        at com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(Unknown Source)
        at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(Unknown Source)
        at javafx.beans.property.ReadOnlyBooleanPropertyBase.fireValueChangedEvent(Unknown Source)
        at javafx.scene.Node$FocusedProperty.notifyListeners(Unknown Source)
        at javafx.scene.Scene$13.invalidated(Unknown Source)
        at javafx.beans.property.ObjectPropertyBase.markInvalid(Unknown Source)
        at javafx.beans.property.ObjectPropertyBase.set(Unknown Source)
        at javafx.scene.Scene$KeyHandler.setFocusOwner(Unknown Source)
        at javafx.scene.Scene$KeyHandler.requestFocus(Unknown Source)
        at javafx.scene.Scene$KeyHandler.access$1900(Unknown Source)
        at javafx.scene.Scene.requestFocus(Unknown Source)
        at javafx.scene.Node.requestFocus(Unknown Source)
        at com.sun.javafx.scene.control.behavior.TextAreaBehavior.mousePressed(Unknown Source)
        at com.sun.javafx.scene.control.skin.TextAreaSkin$ContentView.lambda$new$225(Unknown Source)
        at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(Unknown Source)
        at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(Unknown Source)
        at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
        at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
        at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(Unknown Source)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
        at com.sun.javafx.event.EventUtil.fireEventImpl(Unknown Source)
        at com.sun.javafx.event.EventUtil.fireEvent(Unknown Source)
        at javafx.event.Event.fireEvent(Unknown Source)
        at javafx.scene.Scene$MouseHandler.process(Unknown Source)
        at javafx.scene.Scene$MouseHandler.access$1500(Unknown Source)
        at javafx.scene.Scene.impl_processMouseEvent(Unknown Source)
        at javafx.scene.Scene$ScenePeerListener.mouseEvent(Unknown Source)
        at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(Unknown Source)
        at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$354(Unknown Source)
        at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(Unknown Source)
        at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(Unknown Source)
        at com.sun.glass.ui.View.handleMouseEvent(Unknown Source)
        at com.sun.glass.ui.View.notifyMouse(Unknown Source)
        at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
        at com.sun.glass.ui.win.WinApplication.lambda$null$148(Unknown Source)
        at java.lang.Thread.run(Unknown Source)