FULL PRODUCT VERSION :
java version "1.5.0_06"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_06-b05)
Java HotSpot(TM) Client VM (build 1.5.0_06-b05, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
If a Serializable class defines a readResolve method; then if it refines the return type, the serialization machinery will not locate the method.

This could be a serious bug if a class is written expecting common behavior, but a test is not done; in other words:

public final class Foo implements Serializable {

    private static final Foo SINGLETON = new Foo();
    private Foo() {}
    private Object readResolve() { return SINGLETON; }

}

may be considered an "obvious or easy" class or a very simple type; but if the readResolve method is made covariant:

    private Foo readResolve() { return SINGLETON; }

then the class will be broken -- serialization will not invoke this readResolve method.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
If a Serializable class defines a readResolve method then it must not refine the return type of the method or the seialization machinery will not invoke the method.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The ability to refine the return type of readResolve.
ACTUAL -
An obscured method signature.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
** There is no indication unless testing is performed.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

public class SerTest implements Serializable {
    
    private static final SerTest SINGLETON = new SerTest();

    public static void main(String[] args) throws Exception {
        ByteArrayOutputStream b = new ByteArrayOutputStream();
        (new ObjectOutputStream(b)).writeObject(SINGLETON);
        SerTest re
                = (SerTest) (new ObjectInputStream(
                        new ByteArrayInputStream(
                                b.toByteArray()))).readObject();
        System.out.println(SINGLETON);
        System.out.println(re);
        System.out.println(SINGLETON == re);
    }

    private SerTest() { }
    private SerTest readResolve() { return SerTest.SINGLETON; }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
The method cannot be made covariant.  And in truth, there may not be very much utility in refining the return type; but I'll add again that it may be very easy to write a broken implementation that only test coverage could uncover -- there is no checked warning or any other indication.