When resizing a PieChart, additionnal lines are generated and remain visibile in the node display (see attached screenshot).

Test code:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.chart.PieChart;
import javafx.scene.chart.PieChart.Data;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;

/**
 *
 * @author fabriceb
 */
public class Main extends Application {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Application.launch(Main.class, args);
    }
    
    @Override
    public void start(Stage primaryStage) {
        PieChart pieChart = new PieChart();
        pieChart.getData().addAll(new Data("LINUX", 15), new Data("WINNT51", 40), new Data("WINNT50", 5), new Data("WINNT61", 40));
        primaryStage.setTitle("Pie Trouble");
        BorderPane root = new BorderPane();
        root.setCenter(pieChart);
        Scene scene = new Scene(root, 300, 250);
        primaryStage.setScene(scene);
        primaryStage.setVisible(true);
    }
}

Note: the PieChart displays fine initially, you have to resize the window to have the additionnal line appear.