diff --git a/common/cpw/mods/fml/common/patcher/ClassPatch.java b/common/cpw/mods/fml/common/patcher/ClassPatch.java
index d5d6e7ff6e..833a6ab78e 100644
--- a/common/cpw/mods/fml/common/patcher/ClassPatch.java
+++ b/common/cpw/mods/fml/common/patcher/ClassPatch.java
@@ -6,12 +6,14 @@ public class ClassPatch {
     public final String targetClassName;
     public final boolean existsAtTarget;
     public final byte[] patch;
-    public ClassPatch(String name, String sourceClassName, String targetClassName, boolean existsAtTarget, byte[] patch)
+    public final int inputChecksum;
+    public ClassPatch(String name, String sourceClassName, String targetClassName, boolean existsAtTarget, int inputChecksum, byte[] patch)
     {
         this.name = name;
         this.sourceClassName = sourceClassName;
         this.targetClassName = targetClassName;
         this.existsAtTarget = existsAtTarget;
+        this.inputChecksum = inputChecksum;
         this.patch = patch;
     }
 
diff --git a/common/cpw/mods/fml/common/patcher/ClassPatchManager.java b/common/cpw/mods/fml/common/patcher/ClassPatchManager.java
index be0439f714..33b1d3ec9e 100644
--- a/common/cpw/mods/fml/common/patcher/ClassPatchManager.java
+++ b/common/cpw/mods/fml/common/patcher/ClassPatchManager.java
@@ -26,6 +26,7 @@ import com.google.common.base.Strings;
 import com.google.common.base.Throwables;
 import com.google.common.collect.ArrayListMultimap;
 import com.google.common.collect.ListMultimap;
+import com.google.common.hash.Hashing;
 import com.google.common.io.ByteArrayDataInput;
 import com.google.common.io.ByteStreams;
 import com.google.common.io.Files;
@@ -80,6 +81,14 @@ public class ClassPatchManager {
             {
                 FMLLog.warning("Patcher expecting empty class data file for %s, but received non-empty", patch.targetClassName);
             }
+            else
+            {
+                int inputChecksum = Hashing.adler32().hashBytes(inputData).asInt();
+                if (patch.inputChecksum != inputChecksum)
+                {
+                    FMLLog.severe("There is a binary discrepency between the expected input class %s (%s) and the actual class. Checksum on disk is %x, in patch %x. Things are probably about to go very wrong. Did you put something into the jar file?", mappedName, name, inputChecksum, patch.inputChecksum);
+                }
+            }
             synchronized (patcher)
             {
                 try
@@ -181,10 +190,15 @@ public class ClassPatchManager {
         String sourceClassName = input.readUTF();
         String targetClassName = input.readUTF();
         boolean exists = input.readBoolean();
+        int inputChecksum = 0;
+        if (exists)
+        {
+            inputChecksum = input.readInt();
+        }
         int patchLength = input.readInt();
         byte[] patchBytes = new byte[patchLength];
         input.readFully(patchBytes);
 
-        return new ClassPatch(name, sourceClassName, targetClassName, exists, patchBytes);
+        return new ClassPatch(name, sourceClassName, targetClassName, exists, inputChecksum, patchBytes);
     }
 }
diff --git a/common/cpw/mods/fml/common/patcher/GenDiffSet.java b/common/cpw/mods/fml/common/patcher/GenDiffSet.java
index 410b58768d..9ba3ba512f 100644
--- a/common/cpw/mods/fml/common/patcher/GenDiffSet.java
+++ b/common/cpw/mods/fml/common/patcher/GenDiffSet.java
@@ -13,6 +13,7 @@ import java.util.logging.Logger;
 
 import com.google.common.collect.Iterables;
 import com.google.common.collect.Iterators;
+import com.google.common.hash.Hashing;
 import com.google.common.io.ByteArrayDataOutput;
 import com.google.common.io.ByteStreams;
 import com.google.common.io.Files;
@@ -64,6 +65,7 @@ public class GenDiffSet {
 
                 byte[] diff = delta.compute(vanillaBytes, patchedBytes);
 
+
                 ByteArrayDataOutput diffOut = ByteStreams.newDataOutput(diff.length + 50);
                 // Original name
                 diffOut.writeUTF(name);
@@ -73,6 +75,10 @@ public class GenDiffSet {
                 diffOut.writeUTF(targetClassName);
                 // exists at original
                 diffOut.writeBoolean(entry != null);
+                if (entry != null)
+                {
+                    diffOut.writeInt(Hashing.adler32().hashBytes(vanillaBytes).asInt());
+                }
                 // length of patch
                 diffOut.writeInt(diff.length);
                 // patch