Gregorian calendar gives a java.lang.IllegalArgumentException only on 12-31-00.
The exception arises even in jdk 1.3.0_01 and also jdk 1.2.2_06.

TEST CASE FROM CUSTOMER
=======================
import java.util.*;

public class GregCalBug {
  protected GregorianCalendar gc;

  public void run() {
    int nYear;

    gc = new GregorianCalendar();
    gc.setLenient(false); /* Bug only manifest if lenient set false */
    try
    {
      gc.set(2001, 1, 1, 1, 0, 0);
      nYear = gc.get(Calendar.YEAR);
    }
    catch (Exception ex)
    {
      System.out.println("We should _not_ be here!");
      System.out.println(ex);
    }
  }

  public static void main(String[] args) {
    System.out.println("This small Java class demonstrates a bug in");
    System.out.println("Solaris JVM JDK 1.2.1_04");
    GregCalBug me = new GregCalBug();
    me.run();
  }
}

Output of the above test case when date is set to 12-31-00.
smala:/home/mala2/gregorian 62 % java -fullversion
java full version "1.3.0_01"
smala:/home/mala2/gregorian 63 % javac GregCalBug.java
smala:/home/mala2/gregorian 64 % java GregCalBug
This small Java class demonstrates a bug in
Solaris JVM JDK 1.2.1_04
We should _not_ be here!
java.lang.IllegalArgumentException
smala:/home/mala2/gregorian 65 %