diff --git a/src/stasis/bufferManager/concurrentBufferManager.c b/src/stasis/bufferManager/concurrentBufferManager.c
index 291c562..b8f131e 100644
--- a/src/stasis/bufferManager/concurrentBufferManager.c
+++ b/src/stasis/bufferManager/concurrentBufferManager.c
@@ -183,7 +183,7 @@ static void deinitTLS(void *tlsp) {
   stasis_buffer_concurrent_hash_t *ch = tls->bm->impl;
 
   Page * p = tls->p;
-  p->id = -1;
+  p->id = -2;
   while(hashtable_test_and_set(ch->ht,p->id, p)) {
     p->id --;
   }
@@ -240,7 +240,12 @@ static inline stasis_buffer_concurrent_hash_tls_t * populateTLS(stasis_buffer_ma
           || (!tls->p->dirty)
         )) {
         // The getStaleAndRemove was not atomic with the hashtable remove, which is OK (but we can't trust tmp anymore...)
-        assert(tmp == tls->p);
+        if(tmp != tls->p) {
+          int copy_count = hashtable_debug_number_of_key_copies(ch->ht, tmp->id);
+          assert(copy_count == 1);
+          assert(tmp == tls->p);
+          abort();
+        }
         // note that we'd like to assert that the page is unpinned here.  However, we can't simply look at p->queue, since another thread could be inside the "spooky" quote below.
         tmp = 0;
         if(tls->p->id >= 0) {
@@ -444,7 +449,7 @@ stasis_buffer_manager_t* stasis_buffer_manager_concurrent_hash_open(stasis_page_
 
   for(pageid_t i = 0; i < stasis_buffer_manager_size; i++) {
     Page *p = stasis_buffer_pool_malloc_page(ch->buffer_pool);
-    stasis_buffer_pool_free_page(ch->buffer_pool, p,-1*i);
+    stasis_buffer_pool_free_page(ch->buffer_pool, p,(-1*i)-2);
     pageSetNode(p,0,0);
     (*pagePinCountPtr(p)) = 1;
     ch->lru->insert(ch->lru, p);  // decrements pin count ptr (setting it to zero)
diff --git a/src/stasis/concurrentHash.c b/src/stasis/concurrentHash.c
index dd3e16f..abbc1f5 100644
--- a/src/stasis/concurrentHash.c
+++ b/src/stasis/concurrentHash.c
@@ -10,6 +10,8 @@
 #include <assert.h>
 #include <stdio.h>
 
+//#define STASIS_HASHTABLE_FSCK_THREAD
+
 struct bucket_t {
   pageid_t key;
   pthread_mutex_t mut;
@@ -19,6 +21,10 @@ struct bucket_t {
 struct hashtable_t {
   bucket_t* buckets;
   pageid_t maxbucketid;
+#ifdef STASIS_HASHTABLE_FSCK_THREAD
+  int is_open;
+  pthread_t fsck_thread;
+#endif
 };
 
 static inline pageid_t hashtable_wrap(hashtable_t *ht, pageid_t p) {
@@ -55,6 +61,10 @@ static inline pageid_t hashtable_func(hashtable_t *ht, pageid_t key) {
   return hashtable_wrap(ht, hash6432shift(key));
 }
 
+#ifdef STASIS_HASHTABLE_FSCK_THREAD
+void * hashtable_fsck_worker(void * htp);
+#endif
+
 hashtable_t * hashtable_init(pageid_t size) {
   pageid_t newsize = 1;
   for(int i = 0; size; i++) {
@@ -65,22 +75,72 @@ hashtable_t * hashtable_init(pageid_t size) {
 
   ht->maxbucketid = (newsize) - 1;
   ht->buckets = calloc(ht->maxbucketid+1, sizeof(bucket_t));
+  for(int i = 0; i <= ht->maxbucketid; i++) {
+    ht->buckets[i].key = -1;
+  }
   pthread_mutexattr_t attr;
   pthread_mutexattr_init(&attr);
   pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
   for(pageid_t i = 0; i <= ht->maxbucketid; i++) {
     pthread_mutex_init(&(ht->buckets[i].mut), &attr);
   }
-
+#ifdef STASIS_HASHTABLE_FSCK_THREAD
+  ht->is_open = 1;
+  pthread_create(&ht->fsck_thread,0, hashtable_fsck_worker, ht);
+#endif
   return ht;
 }
 void hashtable_deinit(hashtable_t * ht) {
-  for(pageid_t i = 0; i < ht->maxbucketid; i++) {
+#ifdef STASIS_HASHTABLE_FSCK_THREAD
+  ht->is_open = 0;
+  pthread_join(ht->fsck_thread, 0);
+#endif
+  for(pageid_t i = 0; i <= ht->maxbucketid; i++) {
     pthread_mutex_destroy(&ht->buckets[i].mut);
   }
   free(ht->buckets);
   free(ht);
 }
+
+int hashtable_debug_number_of_key_copies(hashtable_t *ht, pageid_t pageid) {
+  int count = 0;
+  for(int i = 0; i <= ht->maxbucketid; i++) {
+    if(ht->buckets[i].key == pageid) { count ++; }
+  }
+  if(count > 0) { fprintf(stderr, "%d copies of key %lld in hashtable!", count, (unsigned long long) pageid); }
+  return count;
+}
+
+void hashtable_fsck(hashtable_t *ht) {
+  pthread_mutex_lock(&ht->buckets[0].mut);
+  for(int i = 1; i <= ht->maxbucketid; i++) {
+    pthread_mutex_lock(&ht->buckets[i].mut);
+    if(ht->buckets[i].key != -1) {
+      pageid_t this_hash_code = hashtable_func(ht, ht->buckets[i].key);
+      if(this_hash_code != i) {
+        assert(ht->buckets[i-1].key != -1);
+        assert(ht->buckets[i-1].val != 0);
+        assert(this_hash_code < i || (this_hash_code > i + (ht->maxbucketid/2)));
+      }
+    } else {
+      assert(ht->buckets[i].val == NULL);
+    }
+    pthread_mutex_unlock(&ht->buckets[i-1].mut);
+  }
+  pthread_mutex_lock(&ht->buckets[0].mut);
+  if(ht->buckets[0].key != -1) {
+    pageid_t this_hash_code = hashtable_func(ht, ht->buckets[0].key);
+    if(this_hash_code != 0) {
+      assert(ht->buckets[ht->maxbucketid].key != -1);
+      assert(ht->buckets[ht->maxbucketid].val != 0);
+      assert(this_hash_code < 0 || (this_hash_code > 0 + (ht->maxbucketid/2)));
+    }
+  } else {
+    assert(ht->buckets[ht->maxbucketid].val == NULL);
+  }
+  pthread_mutex_unlock(&ht->buckets[ht->maxbucketid].mut);
+  pthread_mutex_unlock(&ht->buckets[0].mut);
+}
 typedef enum {
   LOOKUP,
   INSERT,
@@ -89,6 +149,7 @@ typedef enum {
 } hashtable_mode;
 static inline void * hashtable_begin_op(hashtable_mode mode, hashtable_t *ht, pageid_t p, void *val, hashtable_bucket_handle_t *h) {
   static int warned = 0;
+  assert(p != -1);
   pageid_t idx = hashtable_func(ht, p);
   void * ret;
   bucket_t *b1 = &ht->buckets[idx], *b2 = NULL;
@@ -105,8 +166,8 @@ static inline void * hashtable_begin_op(hashtable_mode mode, hashtable_t *ht, pa
     }
     assert(num_incrs < (ht->maxbucketid/4));
     num_incrs++;
-    if(b1->key == p) { ret = b1->val; break; }
-    if(b1->val == NULL) { ret = NULL; break; }
+    if(b1->key == p) { assert(b1->val); ret = b1->val; break; }
+    if(b1->val == NULL) { assert(b1->key == -1); ret = NULL; break; }
     idx = hashtable_wrap(ht, idx+1);
     b2 = b1;
     b1 = &ht->buckets[idx];
@@ -120,6 +181,20 @@ static inline void * hashtable_begin_op(hashtable_mode mode, hashtable_t *ht, pa
   return ret;
 }
 
+
+#ifdef STASIS_HASHTABLE_FSCK_THREAD
+void * hashtable_fsck_worker(void * htp) {
+  hashtable_t * ht = htp;
+  while(ht->is_open) {
+    fprintf(stderr, "Scanning hashtable %x", (unsigned int)ht);
+    sleep(1);
+    hashtable_fsck(ht);
+  }
+  return 0;
+}
+#endif
+
+
 void hashtable_end_op(hashtable_mode mode, hashtable_t *ht, void *val, hashtable_bucket_handle_t *h) {
   pageid_t idx = h->idx;
   bucket_t * b1 = h->b1;
@@ -142,7 +217,8 @@ void hashtable_end_op(hashtable_mode mode, hashtable_t *ht, void *val, hashtable
       // Case 1: It is null, we win.
       if(b1->val == NULL) {
 //        printf("d\n"); fflush(0);
-        b2->key = 0;
+        assert(b1->key == -1);
+        b2->key = -1;
         b2->val = NULL;
         break;
       } else {
diff --git a/stasis/concurrentHash.h b/stasis/concurrentHash.h
index 494e7c4..49db084 100644
--- a/stasis/concurrentHash.h
+++ b/stasis/concurrentHash.h
@@ -36,6 +36,11 @@ void * hashtable_remove_begin(hashtable_t *ht, pageid_t p, hashtable_bucket_hand
 void   hashtable_remove_finish(hashtable_t *ht, hashtable_bucket_handle_t *h);
 void   hashtable_remove_cancel(hashtable_t *ht, hashtable_bucket_handle_t *h);
 
+/**
+ * @return -0 if key not found, 1 if the key exists, >1 if the hashtable is corrupt, and the key appears multiple times..
+ */
+int hashtable_debug_number_of_key_copies(hashtable_t *ht, pageid_t pageied);
+
 void hashtable_unlock(hashtable_bucket_handle_t *h);
 
 #endif /* CONCURRENTHASH_H_ */