diff --git a/src/allmydata/immutable/checker.py b/src/allmydata/immutable/checker.py
index c565bc3..77edff8 100644
--- a/src/allmydata/immutable/checker.py
+++ b/src/allmydata/immutable/checker.py
@@ -159,7 +159,7 @@ class Checker(log.PrefixingLogMixin):
             self._share_hash_tree = hashtree.IncompleteHashTree(self._verifycap.total_shares)
             self._share_hash_tree.set_hashes({0: vup.share_root_hash})
 
-            vrbp = download.ValidatedReadBucketProxy(sharenum, b, self._share_hash_tree, vup.num_segments, vup.block_size, vup.share_size)
+            vrbp = download.ValidatedReadBucketProxy(sharenum, b, self._share_hash_tree, vup.num_segments, vup.block_size, vup.share_size, self._verifycap.storage_index)
 
             ds = []
             for blocknum in range(vup.num_segments):
diff --git a/src/allmydata/immutable/download.py b/src/allmydata/immutable/download.py
index 9dfc0bb..efb5454 100644
--- a/src/allmydata/immutable/download.py
+++ b/src/allmydata/immutable/download.py
@@ -318,7 +318,8 @@ class ValidatedReadBucketProxy(log.PrefixingLogMixin):
     My get_block() method is used by BlockDownloaders.
     """
 
-    def __init__(self, sharenum, bucket, share_hash_tree, num_blocks, block_size, share_size):
+    def __init__(self, sharenum, bucket, share_hash_tree, num_blocks, block_size, share_size,
+                 storage_index):
         """ share_hash_tree is required to have already been initialized with the root hash
         (the number-0 hash), using the share_root_hash from the UEB """
         precondition(share_hash_tree[0] is not None, share_hash_tree)
@@ -331,6 +332,7 @@ class ValidatedReadBucketProxy(log.PrefixingLogMixin):
         self.block_size = block_size
         self.share_size = share_size
         self.block_hash_tree = hashtree.IncompleteHashTree(self.num_blocks)
+        self.storage_index = storage_index
 
     def get_block(self, blocknum):
         # the first time we use this bucket, we need to fetch enough elements
@@ -408,6 +410,15 @@ class ValidatedReadBucketProxy(log.PrefixingLogMixin):
             # likely a programming error
             self.log("hash failure in block=%d, shnum=%d on %s" %
                     (blocknum, self.sharenum, self.bucket))
+            try:
+                fn = ("logs/badhash-from-%s-SI-%s-shnum-%d-blocknum-%d" %
+                      (idlib.shortnodeid_b2a(self.bucket._peerid),
+                       base32.b2a(self.storage_index), self.sharenum, blocknum))
+                f = open(fn, "wb")
+                f.write(blockdata)
+                f.close()
+            except Exception, e:
+                self.log(" (unable to write blockdata: %s)" % (e,))
             if self.block_hash_tree.needed_hashes(blocknum):
                 self.log(""" failure occurred when checking the block_hash_tree.
                 This suggests that either the block data was bad, or that the
@@ -923,7 +934,10 @@ class CiphertextDownloader(log.PrefixingLogMixin):
 
     def _download_all_segments(self, res):
         for sharenum, bucket in self._share_buckets:
-            vbucket = ValidatedReadBucketProxy(sharenum, bucket, self._share_hash_tree, self._vup.num_segments, self._vup.block_size, self._vup.share_size)
+            vbucket = ValidatedReadBucketProxy(sharenum, bucket, self._share_hash_tree,
+                                               self._vup.num_segments,
+                                               self._vup.block_size, self._vup.share_size,
+                                               self._storage_index)
             self._share_vbuckets.setdefault(sharenum, set()).add(vbucket)
 
         # after the above code, self._share_vbuckets contains enough