diff --git a/src/main/java/org/apache/hadoop/hbase/io/HbaseObjectWritable.java b/src/main/java/org/apache/hadoop/hbase/io/HbaseObjectWritable.java
index 90dfb843f38..260f982a426 100644
--- a/src/main/java/org/apache/hadoop/hbase/io/HbaseObjectWritable.java
+++ b/src/main/java/org/apache/hadoop/hbase/io/HbaseObjectWritable.java
@@ -123,6 +123,14 @@ public class HbaseObjectWritable implements Writable, WritableWithSize, Configur
   // sending of the class name using reflection, etc.
   private static final byte NOT_ENCODED = 0;
   static {
+    ////////////////////////////////////////////////////////////////////////////
+    // WARNING: Please do not insert, remove or swap any line in this static  //
+    // block.  Doing so would change or shift all the codes used to serialize //
+    // objects, which makes backwards compatibility very hard for clients.    //
+    // New codes should always be added at the end. Code removal is           //
+    // discouraged because code is a short now.                               //
+    ////////////////////////////////////////////////////////////////////////////
+
     int code = NOT_ENCODED + 1;
     // Primitive types.
     addToMap(Boolean.TYPE, code++);
@@ -165,7 +173,6 @@ public class HbaseObjectWritable implements Writable, WritableWithSize, Configur
     addToMap(HServerAddress.class, code++);
     addToMap(HServerInfo.class, code++);
     addToMap(HTableDescriptor.class, code++);
-    addToMap(HTableDescriptor[].class, code++);
     addToMap(MapWritable.class, code++);
 
     //
@@ -222,7 +229,7 @@ public class HbaseObjectWritable implements Writable, WritableWithSize, Configur
     addToMap(Increment.class, code++);
 
     addToMap(KeyOnlyFilter.class, code++);
-    
+
     // serializable
     addToMap(Serializable.class, code++);
 
@@ -236,6 +243,8 @@ public class HbaseObjectWritable implements Writable, WritableWithSize, Configur
 
     addToMap(RegionOpeningState.class, code++);
 
+    addToMap(HTableDescriptor[].class, code++);
+    
     addToMap(Append.class, code++);
 
     addToMap(RowMutation.class, code++);
@@ -593,7 +602,7 @@ public class HbaseObjectWritable implements Writable, WritableWithSize, Configur
         in.readFully(objectBytes);
         ByteArrayInputStream bis = null;
         ObjectInputStream ois = null;
-        try { 
+        try {
           bis = new ByteArrayInputStream(objectBytes);
           ois = new ObjectInputStream(bis);
           instance = ois.readObject();
diff --git a/src/main/java/org/apache/hadoop/hbase/ipc/HBaseServer.java b/src/main/java/org/apache/hadoop/hbase/ipc/HBaseServer.java
index c2cacd97664..e20ec4346e2 100644
--- a/src/main/java/org/apache/hadoop/hbase/ipc/HBaseServer.java
+++ b/src/main/java/org/apache/hadoop/hbase/ipc/HBaseServer.java
@@ -1213,9 +1213,10 @@ public abstract class HBaseServer implements RpcServer {
       header.readFields(in);
       try {
         String protocolClassName = header.getProtocol();
-        if (protocolClassName != null) {
-          protocol = getProtocolClass(header.getProtocol(), conf);
+        if (protocolClassName == null) {
+          protocolClassName = "org.apache.hadoop.hbase.ipc.HRegionInterface";
         }
+        protocol = getProtocolClass(protocolClassName, conf);
       } catch (ClassNotFoundException cnfe) {
         throw new IOException("Unknown protocol: " + header.getProtocol());
       }
diff --git a/src/main/java/org/apache/hadoop/hbase/ipc/Invocation.java b/src/main/java/org/apache/hadoop/hbase/ipc/Invocation.java
index 39c73f53fb6..055787e560d 100644
--- a/src/main/java/org/apache/hadoop/hbase/ipc/Invocation.java
+++ b/src/main/java/org/apache/hadoop/hbase/ipc/Invocation.java
@@ -22,6 +22,7 @@ package org.apache.hadoop.hbase.ipc;
 import org.apache.hadoop.conf.Configurable;
 import org.apache.hadoop.conf.Configuration;
 import org.apache.hadoop.hbase.io.HbaseObjectWritable;
+import org.apache.hadoop.io.VersionMismatchException;
 import org.apache.hadoop.io.VersionedWritable;
 
 import java.io.DataInput;
@@ -94,10 +95,29 @@ public class Invocation extends VersionedWritable implements Configurable {
   }
 
   public void readFields(DataInput in) throws IOException {
-    super.readFields(in);
-    methodName = in.readUTF();
-    clientVersion = in.readLong();
-    clientMethodsHash = in.readInt();
+    try {
+      super.readFields(in);
+      methodName = in.readUTF();
+      clientVersion = in.readLong();
+      clientMethodsHash = in.readInt();
+    } catch (VersionMismatchException e) {
+      // VersionMismatchException doesn't provide an API to access
+      // expectedVersion and foundVersion.  This is really sad.
+      if (e.toString().endsWith("found v0")) {
+        // Try to be a bit backwards compatible.  In previous versions of
+        // HBase (before HBASE-3939 in 0.92) Invocation wasn't a
+        // VersionedWritable and thus the first thing on the wire was always
+        // the 2-byte length of the method name.  Because no method name is
+        // longer than 255 characters, and all method names are in ASCII,
+        // The following code is equivalent to `in.readUTF()', which we can't
+        // call again here, because `super.readFields(in)' already consumed
+        // the first byte of input, which can't be "unread" back into `in'.
+        final short len = (short) (in.readByte() & 0xFF);  // Unsigned byte.
+        final byte[] buf = new byte[len];
+        in.readFully(buf, 0, len);
+        methodName = new String(buf);
+      }
+    }
     parameters = new Object[in.readInt()];
     parameterClasses = new Class[parameters.length];
     HbaseObjectWritable objectWritable = new HbaseObjectWritable();