J2SE API Document read:

IllegalAccessException -
    if the class or its nullary constructor is not accessible.
InstantiationException -
    if this Class represents an abstract class, an interface, an array class,
    a primitive type, or void; or if the class has no nullary constructor;
    or if the instantiation fails for some other reason.

Following simple test throw InstantiationException in ladybird.
bug throw IllegalAccessException in merlin.

abstract class Y
{
   private Y() { }
}

class test1
{
   test1() { }
   public static void main( String argv[] )
   {
      Y oy;
      try {
        oy = (Y) Class.forName("Y").newInstance();
      } catch (Exception e) {
        System.out.println (e);
      }
   }
}

$ /import/java/jdk1.3.1/solsparc/bin/java test1
java.lang.InstantiationException: Y

$ /import/java/jdk1.4/solsparc/bin/java test1
java.lang.IllegalAccessException: Class test1 can not access a member of class Y with modifiers "private"