It maybe that the appending object and the object your appending to needs to
implement the serializable interface.

Sharp Tool

See http://mindprod.com/jgloss/gotchas.html#SERIALIZATION
and
http://mindprod.com/jgloss/runerrormessages.html#STREAMCORRUPTEDEXCEPTION

I wrote myself a little SSCCE to investigate appending.  I generated
it with the file I/O Amanuensis to help avoid clerical errors.

See http://mindprod.com/applets/fileio.html

// test ability to append to a serialized object stream
import java.io.*;

public class ObjAppend
  {

  /**
   * write some string objects to serialized file
   */
  private static void write1() throws IOException
  {
     // O P E N
     FileOutputStream fos = new FileOutputStream( "C:/temp/temp.ser",
false /* append */ );
     BufferedOutputStream bos = new BufferedOutputStream( fos, 4096
/* buffsize */ );
     ObjectOutputStream oos = new ObjectOutputStream( bos );

     // W R I T E
     oos.writeObject( "a string" );
     oos.writeObject( "a second string" );
     oos.reset();
     oos.writeObject( "a third string" );

     oos.flush();

     // C L O S E
     oos.close();
  }

  /**
   * Append a string object to serialized file
   */
  private static void write2() throws IOException
  {
     // O P E N
     FileOutputStream fos = new FileOutputStream( "C:/temp/temp.ser",
true /* append */ );
     BufferedOutputStream bos = new BufferedOutputStream( fos, 4096
/* buffsize */ );
     ObjectOutputStream oos = new ObjectOutputStream( bos );

     // W R I T E

     oos.writeObject( "a fourth string, appended" );
     oos.flush();

     // C L O S E
     oos.close();
  }

  /**
   * read back four strings from serialized file
   */
  private static void read1() throws IOException,
ClassNotFoundException
  {
     // O P E N
     FileInputStream fis = new FileInputStream( "C:/temp/temp.ser" );
     BufferedInputStream bis = new BufferedInputStream( fis, 4096 /*
buffsize */ );
     ObjectInputStream ois = new ObjectInputStream( bis );

     // R E A D   four strings
     for ( int i=0; i<4; i++ )
        {
        System.out.println( (String) ois.readObject() );
        }

     // C L O S E
     ois.close();
  }
  /**
   * test ability to append to a serialiszed object stream
   *
   * @param args not used
   */
  public static void main ( String[] args )
     {
     try
        {
        write1();
        write2();
        read1();
        }
     catch ( Exception e )
        {
        e.printStackTrace();
        }

     }  // end main
  }
---------------------------------

It dies with the following, output just like yours reading the
appended string.

a string
a second string
a third string
java.io.StreamCorruptedException
       at java.io.ObjectInputStream.readObject0(Unknown Source)
       at java.io.ObjectInputStream.readObject(Unknown Source)
       at ObjAppend.read1(ObjAppend.java:63)
       at ObjAppend.main(ObjAppend.java:80)

The key to why this fails can be found by examining the file
c:\temp\temp.ser with a hex viewer.  See
http://mindprod.com/jgloss/hex.html

In short, what gets written to the file on an append is not the same
as when you do a reset.  readObject is not smart enough to ignore
extra stream init stuff in the middle of a stream.

So the short answer is, you can't append. You must write to a new
file, or copy and write to a new file.