ADDITIONAL SYSTEM INFORMATION :
Microsoft Windows [Version 10.0.17134.1040]
openjdk version "13" 2019-09-17
OpenJDK Runtime Environment AdoptOpenJDK (build 13+33)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 13+33, mixed mode, sharing)

A DESCRIPTION OF THE PROBLEM :
Adding children to wrong index leaves inconsistent state in Parent#childrenSet, so the next call to remove throws exception. Run attached sources.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run provided test case.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Exception should not be thrown on removing the item.
ACTUAL -
Exception is thrown on removing the item.

---------- BEGIN SOURCE ----------
import javafx.application.Application;
import javafx.scene.control.Label;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class TestFx_collectionInconsistentState
{

    public static void main( String[] args )
    {
        Application.launch( FxApp.class, args );
    }

    public static class FxApp extends Application
    {

        @Override
        public void start( final Stage primaryStage ) throws Exception
        {
            System.err.println( System.getProperty( "javafx.version" ) );
            final var pane = new VBox();
            pane.getChildren().addAll( new Label( "1" ), new Label( "2" ) );
            try
            {
                pane.getChildren().add( -1, new Label( "0" ) );
            }
            catch( IndexOutOfBoundsException aE )
            {
                // Exception is thrown, but the list state becomes corrupted
            }
            // But javafx.scene.Parent.childSet does not revert its state
            pane.getChildren().remove( 0 );
        }
    }

}

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

FREQUENCY : always