test_descr.py crashes under the debug build now. I've got too many things piled on my stack to look into it right now, so it would be good if someone could try to feel debug pain on Linux too. The top of the call stack is _free_dbg_lk(void * 0x009e5fd0, int 1) line 1066 + 60 bytes _free_dbg(void * 0x009e5fd0, int 1) line 1001 + 13 bytes free(void * 0x009e5fd0) line 956 + 11 bytes _PyObject_GC_Del(_object * 0x009e5fdc) line 873 + 10 bytes long_dealloc(_object * 0x009e5fdc) line 1230 + 16 bytes subtype_dealloc(_object * 0x009e5fdc) line 301 + 7 bytes _Py_Dealloc(_object * 0x009e5fdc) line 1738 + 7 bytes eval_frame(_frame * 0x00803d7c) line 845 + 59 bytes PyEval_EvalCodeEx(...) at this point, so first guess is that it has to do with adding new-style instances to GC. The MS debug library free() is complaining because it detects damage (corruption) in the pad bytes debug malloc() writes *between* the blocks it hands out. So it's unlikely that it points to the direct cause; it's just detecting that user code, somewhere, sometime, wrote over memory it had no business writing to.