Execute the test below - the listener should be called 30 times but for some reason the number of calls varies from 1 to 30, mostly it's 5-15. The test works fine on Java 7u45 and Java 8b81. I don't know exactly which release introduced the issue - it does not work with Java 8b99. The issue also does not occur when you remark the line fxmlLoader.load().

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

  @Override
  public void start(Stage stage)
  {
    final StackPane root = new StackPane();

    Button b = new Button("Add Panes");
    b.setOnAction(new EventHandler<ActionEvent>()
    {
      @Override
      public void handle(ActionEvent event)
      {
        for (int i=0; i<30; i++)
        {
          MyPane myPane = new MyPane(root, i);
          root.getChildren().add(myPane);
        }
      }
    });
    root.getChildren().add(b);
    
    Scene scene = new Scene(root, 800, 600);
    stage.setScene(scene);
    stage.show();
  }

  private static class MyPane extends Pane
  {
    public MyPane(Pane parent, final int i)
    {
      try
      {
        FXMLLoader fxmlLoader = new FXMLLoader(getClass().getResource("/test/bind/test.fxml"));
        //when the line below is remarked the issue does not occur
        fxmlLoader.load();
      }
      catch (Exception e)
      {
        e.printStackTrace();
      }

      final DoubleProperty x = new SimpleDoubleProperty();
      x.bind(widthProperty());
      x.addListener(new ChangeListener<Number>()
      {
        @Override
        public void changed(ObservableValue<? extends Number> obs, Number oldValue, Number newValue)
        {
          System.err.println(" x" + i + " " + newValue);
        }
      });
      
      getChildren().add(new Button("Test"));
      setMouseTransparent(true);
    }
  }
}

test.fxml:
<?xml version="1.0" encoding="UTF-8"?>

<?import java.lang.*?>
<?import java.util.*?>
<?import javafx.geometry.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.paint.*?>

<GridPane id="rootPaneTitlePane" alignment="TOP_LEFT" gridLinesVisible="false" hgap="0.0" layoutY="0.0" minHeight="-1.0" prefHeight="-1.0" prefWidth="-1.0" xmlns:fx="http://javafx.com/fxml">
  <children>
    <Label fx:id="titleLabel" alignment="CENTER" contentDisplay="LEFT" prefWidth="-1.0" text="title" textAlignment="CENTER" wrapText="false" GridPane.columnIndex="0" GridPane.halignment="CENTER" GridPane.hgrow="NEVER" GridPane.rowIndex="0">
      <GridPane.margin>
        <Insets fx:id="x1" />
      </GridPane.margin>
    </Label>
    <Button fx:id="closeButton" graphicTextGap="0.0" maxWidth="-1.0" minHeight="-1.0" minWidth="-1.0" mnemonicParsing="false" prefHeight="-1.0" prefWidth="-1.0" text="" GridPane.columnIndex="1" GridPane.margin="$x1" GridPane.rowIndex="0" />
  </children>
  <columnConstraints>
    <ColumnConstraints fillWidth="true" hgrow="ALWAYS" maxWidth="-Infinity" minWidth="-1.0" prefWidth="-1.0" />
    <ColumnConstraints hgrow="SOMETIMES" maxWidth="-Infinity" minWidth="-Infinity" prefWidth="-1.0" />
  </columnConstraints>
  <rowConstraints>
    <RowConstraints minHeight="-Infinity" vgrow="SOMETIMES" />
  </rowConstraints>
</GridPane>