Index: src/btree.c
==================================================================
--- src/btree.c
+++ src/btree.c
@@ -627,15 +627,49 @@
     sqlite3_free(pMap->aSvpt);
     sqlite3_free(pMap);
     pBt->pMap = 0;
   }
 }
+
+static void btreeCheckPtrmap(BtShared *p, int nPage, const char *zLog){
+  BtreePtrmap *pMap = p->pMap;
+  if( pMap ){
+    int n = MIN(1 + nPage - (int)pMap->iFirst, 5);
+    int i;
+    for(i=0; i<n; i++){
+      int eType = pMap->aPtr[i].eType;
+      if( (eType==PTRMAP_OVERFLOW1 || 
+            eType==PTRMAP_OVERFLOW2 || 
+            eType==PTRMAP_BTREE) && pMap->aPtr[i].parent==0 
+        ){
+        sqlite3_log(SQLITE_ERROR, 
+            "Bitvec: error at (%s) - (%d/%d %d/%d %d/%d %d/%d %d/%d)",
+            zLog,
+            (int)pMap->aPtr[0].eType, (int)pMap->aPtr[0].parent,
+            (n>1 ? (int)pMap->aPtr[1].eType : -1),
+            (n>1 ? (int)pMap->aPtr[1].parent : -1),
+
+            (n>2 ? (int)pMap->aPtr[2].eType : -1),
+            (n>2 ? (int)pMap->aPtr[2].parent : -1),
+
+            (n>3 ? (int)pMap->aPtr[3].eType : -1),
+            (n>3 ? (int)pMap->aPtr[3].parent : -1),
+
+            (n>4 ? (int)pMap->aPtr[4].eType : -1),
+            (n>4 ? (int)pMap->aPtr[4].parent : -1)
+            );
+        break;
+      }
+    }
+  }
+}
 #else  /* SQLITE_OMIT_CONCURRENT */
 # define btreePtrmapAllocate(x) SQLITE_OK
 # define btreePtrmapDelete(x) 
 # define btreePtrmapBegin(x,y)  SQLITE_OK
 # define btreePtrmapEnd(x,y,z) 
+# define btreeCheckPtrmap(a,b,c)
 #endif /* SQLITE_OMIT_CONCURRENT */
 
 static void releasePage(MemPage *pPage);  /* Forward reference */
 
 /*
@@ -4085,10 +4119,12 @@
   ** required. Otherwise, if page 1 is dirty, proceed.  */
   BtreePtrmap *pMap = pBt->pMap;
   Pgno iTrunk = get4byte(&p1[32]);
   Pgno nPage = btreePagecount(pBt);
   u32 nFree = get4byte(&p1[36]);
+
+  btreeCheckPtrmap(pBt, nPage, "btreeFixUnlocked(1)");
 
   assert( pBt->pMap );
   rc = sqlite3PagerUpgradeSnapshot(pPager, pPage1->pDbPage);
   assert( p1==pPage1->aData );
 
@@ -8672,10 +8708,11 @@
     }
   }
   assert( pCur->apPage[pCur->iPage]->nOverflow==0 );
 
 end_insert:
+  btreeCheckPtrmap(pBt, pBt->nPage, "sqlite3BtreeInsert()");
   return rc;
 }
 
 /*
 ** Delete the entry that the cursor is pointing to. 
@@ -8843,10 +8880,11 @@
       if( bPreserve ){
         pCur->eState = CURSOR_REQUIRESEEK;
       }
     }
   }
+  btreeCheckPtrmap(pBt, pBt->nPage, "sqlite3BtreeInsert()");
   return rc;
 }
 
 /*
 ** Create a new BTree table.  Write into *piTable the page

Index: test/concurrent3.test
==================================================================
--- test/concurrent3.test
+++ test/concurrent3.test
@@ -24,12 +24,12 @@
   return
 }
 
 db close
 sqlite3_shutdown
-#test_sqlite3_log xLog
-#proc xLog {error_code msg} { puts "$error_code: $msg" }
+test_sqlite3_log xLog
+proc xLog {error_code msg} { puts "$error_code: $msg" }
 reset_db
 
 proc create_schema {} {
   db eval {
     PRAGMA journal_mode = wal;