item.h:

  A fix for Bug#6042 "constants propogation works olny once (prepared
  statements)": reset item->marker in Item::cleanup, as it's used
  in propogate_cond_constants. No test case as the only way I could
  come up with to show the problem is EXPLAIN, and EXPLAIN is painful
  to use in the test suite.

sql/item.h

@@ -144,6 +144,7 @@ class Item {
     DBUG_ENTER("Item::cleanup");
     DBUG_PRINT("info", ("Type: %d", (int)type()));
     fixed=0;
+    marker= 0;
     DBUG_VOID_RETURN;
   }
   virtual void make_field(Send_field *field);