Index: lucene/src/java/org/apache/lucene/index/SegmentReader.java
===================================================================
--- lucene/src/java/org/apache/lucene/index/SegmentReader.java	(revision 1088725)
+++ lucene/src/java/org/apache/lucene/index/SegmentReader.java	(working copy)
@@ -441,6 +441,7 @@
       boolean success = false;
       try {
         try {
+          out.writeBytes(SegmentMerger.NORMS_HEADER, 0, SegmentMerger.NORMS_HEADER.length);
           out.writeBytes(bytes, maxDoc());
         } finally {
           out.close();
@@ -986,8 +987,19 @@
           // If this were to change in the future, a clone could be done here.
           normInput = singleNormStream;
         } else {
-          normSeek = 0;
           normInput = d.openInput(fileName);
+          // if the segment was created after 3.1, we wrote the header for sure,
+          // and dont need to do the sketchy file size check. otherwise, we check 
+          // if the size is exactly equal to maxDoc to detect a headerless file.
+          String version = si.getVersion();
+          final boolean isUnversioned = 
+            (version == null || version.equals("3.0") || version.equals("3.1")) 
+            && normInput.length() == maxDoc();
+          if (isUnversioned) {
+            normSeek = 0;
+          } else {
+            normSeek = SegmentMerger.NORMS_HEADER.length;
+          }
         }
 
         norms.put(fi.name, new Norm(normInput, fi.number, normSeek));