ADDITIONAL SYSTEM INFORMATION :
I have replicated this on Java 8u191 and Java 11.0.2 with JavaFX 11.0.2

A DESCRIPTION OF THE PROBLEM :
Creating animated javafx.scene.image.Image from 2 threads causes ArrayIndexOutOfBoundsException exception.
Issue is caused by the fact that animated Images adds pulse listener without checking if it's invoked from fx thread, so there might be a case where two separate threads breaks pulse listener array in "AbstractMasterTimer" called "receivers".

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Provide all dependencies to attached code (javafx-controls) and "cat.gif" as a resource, which might be any gif animated image with more than 1 frame.
2. Run the code.
3. Exception should be shown in console window
Exception might not occur on every application start, but on 8/10 occurs in a different way.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Toolkit#checkFxUserThread() is invoked while retrieving AbstractMasterTimer from Toolkit#getMasterTimer() and if Image is created on separate thread, an exception is thrown
ACTUAL -
ArrayIndexOutOfBoundsException exception is thrown

---------- BEGIN SOURCE ----------
package com.example;

import java.io.IOException;
import java.io.InputStream;
import java.util.concurrent.CountDownLatch;

import com.sun.javafx.tk.Toolkit;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.image.Image;
import javafx.scene.image.ImageView;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;

public class SimpleMain
{

    public static void main( String[] args ) throws Exception
    {
        final int threadNo = 4;
        final CountDownLatch latch = new CountDownLatch( threadNo + 1 );
        for( int i = 0; i < threadNo; i++ )
        {
            new Thread( () -> {
                latch.countDown();
                try
                {
                    latch.await();
                }
                catch( InterruptedException aE )
                {
                    aE.printStackTrace();
                }
                try
                {
                    createImage();
                }
                catch( Exception aE )
                {
                    aE.printStackTrace();
                }
            } ).start();
        }
        Toolkit.getToolkit().getMasterTimer().addPulseReceiver( now -> {
            latch.countDown();
        } );
        Application.launch( MainFx.class, args );
        latch.await();
    }

    private static Image createImage()
    {
        try (final InputStream catResource = SimpleMain.class.getResourceAsStream( "/cat.gif" ))
        {
            return new Image( catResource );
        }
        catch( IOException aE )
        {
            throw new IllegalStateException( aE );
        }
    }

    public static class MainFx extends Application
    {

        @Override
        public void start( final Stage primaryStage ) throws Exception
        {
            System.err.println( Runtime.version().toString() );
            System.err.println( System.getProperty( "javafx.version" ) );
            System.err.println( System.getProperty( "javafx.runtime.version" ) );
            final int threadNo = 4;
            final CountDownLatch latch = new CountDownLatch( threadNo + 1 );
            for( int i = 0; i < threadNo; i++ )
            {
                new Thread( () -> {
                    latch.countDown();
                    try
                    {
                        latch.await();
                    }
                    catch( InterruptedException aE )
                    {
                        aE.printStackTrace();
                    }
                    try
                    {
                        createImage();
                    }
                    catch( Exception aE )
                    {
                        aE.printStackTrace();
                    }
                } ).start();
            }
            latch.countDown();
            latch.await();
            final Image catImage = createImage();
            final Scene aScene = new Scene( new BorderPane( new ImageView( catImage ) ), 800, 600 );
            primaryStage.setScene( aScene );
            primaryStage.show();
        }

    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Do not create animated images on non-FX thread

FREQUENCY : often