MCP will reobf classes with reserved names in windows to a legal file name. And FML will try to read these classes over the default ones.

fml/commands.patch

@@ -106,25 +106,40 @@
                  for class_file in fnmatch.filter(filelist, '*.class'):
                      class_name = class_path + os.path.splitext(class_file)[0]
                      bin_file = os.path.normpath(os.path.join(path, class_file))
-@@ -1554,7 +1565,16 @@
-                 except KeyError:
-                     self.logger.error('* File %s not found for %s', out_class, in_class)
-                 except IOError:
--                    self.logger.error('* File %s failed extracting for %s', out_class, in_class)
-+                    if not '/' in out_class:
-+                        try:
-+                            f = open(os.path.join(outpathlk[side], '_' + out_class, 'wb'))
-+                            f.write(zipjar.read(out_class))
-+                            f.close()
-+                            self.logger.info('> Outputted %s to %s as %s', in_class.ljust(35), outpathlk[side], '_' + out_class)
-+                        except IOError:
-+                            self.logger.error('* File %s failed extracting for %s', out_class, in_class)
-+                    else:
-+                        self.logger.error('* File %s failed extracting for %s', out_class, in_class)
+@@ -1535,6 +1546,8 @@
+         if not os.path.exists(outpathlk[side]):
+             os.makedirs(outpathlk[side])
  
-     def downloadupdates(self, force=False):
-         if not self.updateurl:
-@@ -1594,6 +1614,9 @@
++        reserved = ['CON', 'PRN', 'AUX', 'NUL', 'COM1', 'COM2', 'COM3', 'COM4', 'COM5', 'COM6', 'COM7', 'COM8', 'COM9', 'LPT1', 'LPT2', 'LPT3', 'LPT4', 'LPT5', 'LPT6', 'LPT7', 'LPT8', 'LPT9']
++
+         # HINT: We extract the modified class files
+         with closing(zipfile.ZipFile(jarlk[side])) as zipjar:
+             for in_class in trgclasses:
+@@ -1548,6 +1561,23 @@
+                     out_class = out_class.replace(self.nullpkg, '')
+                     if out_class[0] == '/':
+                         out_class = out_class[1:]
++                        
++                rename = False
++                for res in reserved:
++                    if out_class.upper().startswith(res):
++                        rename = True
++                        break
++                        
++                if rename:
++                    try:
++                        f = open(os.path.join(outpathlk[side], '_' + out_class), 'wb')
++                        f.write(zipjar.read(out_class))
++                        f.close()
++                        self.logger.info('> Outputted %s to %s as %s', in_class.ljust(35), outpathlk[side], '_' + out_class)
++                    except IOError:
++                        self.logger.error('* File %s failed extracting for %s', out_class, in_class)
++                    continue
++                
+                 try:
+                     zipjar.extract(out_class, outpathlk[side])
+                     self.logger.info('> Outputted %s to %s as %s', in_class.ljust(35), outpathlk[side], out_class)
+@@ -1594,6 +1624,9 @@
                  sys.exit(1)
  
          for entry in newfiles:
@@ -134,7 +149,7 @@
              if entry[3] == 'U':
                  self.logger.info('Retrieving file from server : %s', entry[0])
                  cur_file = os.path.normpath(entry[0])
-@@ -1614,6 +1637,9 @@
+@@ -1614,6 +1647,9 @@
          md5reoblk = {CLIENT: self.md5reobfclient, SERVER: self.md5reobfserver}
          outpathlk = {CLIENT: self.srcmodclient, SERVER: self.srcmodserver}
          src = {CLIENT: self.srcclient, SERVER: self.srcserver}
@@ -144,7 +159,7 @@
  
          # HINT: We need a table for the old md5 and the new ones
          md5table = {}
-@@ -1656,3 +1682,6 @@
+@@ -1656,3 +1692,6 @@
                  except IOError:
                      self.logger.error('* File %s copy failed', in_class)
  

fml/common/cpw/mods/fml/relauncher/RelaunchClassLoader.java

@@ -15,6 +15,7 @@ import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
 import java.util.jar.Attributes.Name;
@@ -40,6 +41,8 @@ public class RelaunchClassLoader extends URLClassLoader
     private Map<Package,Manifest> packageManifests = new HashMap<Package,Manifest>();
 
     private static Manifest EMPTY = new Manifest();
+    
+    private static final String[] RESERVED = {"CON", "PRN", "AUX", "NUL", "COM1", "COM2", "COM3", "COM4", "COM5", "COM6", "COM7", "COM8", "COM9", "LPT1", "LPT2", "LPT3", "LPT4", "LPT5", "LPT6", "LPT7", "LPT8", "LPT9"};
 
     public RelaunchClassLoader(URL[] sources)
     {
@@ -271,6 +274,21 @@ public class RelaunchClassLoader extends URLClassLoader
 
     public byte[] getClassBytes(String name) throws IOException
     {
+        if (name.indexOf('.') == -1)
+        {
+            for (String res : RESERVED)
+            {
+                if (name.toUpperCase(Locale.ENGLISH).startsWith(res))
+                {
+                    byte[] data = getClassBytes("_" + name);
+                    if (data != null)
+                    {
+                        return data;
+                    }
+                }
+            }
+        }
+
         InputStream classStream = null;
         try
         {