If components are added to Container using Container.add(Component comp, int
index), and index is not consecutive, IllegalArgumentException will be thrown.

Here is an example:

import java.awt.*;

public class ContainerAdd {
    public static void main(String[] args) throws Exception {
        Container c = new Container();
        int[]  values = {0, 1, 2, 3, 4, 8}; 
        LWComponent comp = new LWComponent();  
        for(int i=0;i<values.length;i++) {
            try {
                c.add(new LWComponent(), values[i]);
            } catch (ArrayIndexOutOfBoundsException e) {
                System.out.println("Unexpected ArrayIndexOutOfBoundsException, index=" + values[i]);
            } catch (IllegalArgumentException e) {
                System.out.println("IllegalArgumentExeption is thrown for index " + values[i]);
                e.printStackTrace();
            }
        }
    }
}

class LWComponent extends Component {
    public LWComponent() {
    }
}

Output in build 1.4.1-beta-b11 is:

IllegalArgumentExeption is thrown for index 8
java.lang.IllegalArgumentException: illegal component position
        at java.awt.Container.addImpl(Container.java:605)
        at java.awt.Container.add(Container.java:346)
        at ContainerAdd.main(ContainerAdd.java:10)

"IllegalArgumentException - if index is invalid" this is unclear. User needs to know what indices are invalid. 
This behavior needs to be documented.
.