Index: src/java/org/apache/lucene/store/SimpleFSDirectory.java
===================================================================
--- src/java/org/apache/lucene/store/SimpleFSDirectory.java	(revision 1032918)
+++ src/java/org/apache/lucene/store/SimpleFSDirectory.java	(working copy)
@@ -53,7 +53,11 @@
   @Override
   public IndexOutput createOutput(String name) throws IOException {
     initOutput(name);
-    return new SimpleFSIndexOutput(new File(directory, name));
+    File fullPath = new File(directory, name);
+    if (!name.equals("segments.gen") && fullPath.exists()) {
+      throw new RuntimeException("path " + fullPath + " already exists");
+    }
+    return new SimpleFSIndexOutput(fullPath);
   }
 
   /** Creates an IndexInput for the file with the given name. */