A class member that does not appear in the source code must be marked using a Synthetic attribute. This is according to the Java Virtual Machine specification (§4.7.6). Javac did not mark default constructor using a Synthetic attribute.
To see the problem compile following class:

public class Test {
    public void x(){}
}

and check using classfile inspector that <init> method does not have SYNTHETIC attribute. This bug is also in javac 1.3.1-b24