note mutating tp_dict is bad (closes #12719)
diff --git a/Doc/c-api/typeobj.rst b/Doc/c-api/typeobj.rst
index 4a6335c..68ca9ad 100644
--- a/Doc/c-api/typeobj.rst
+++ b/Doc/c-api/typeobj.rst
@@ -745,6 +745,11 @@
This field is not inherited by subtypes (though the attributes defined in here
are inherited through a different mechanism).
+ .. warning::
+
+ It is not safe to use :c:func:`PyDict_SetItem` on or otherwise modify
+ :attr:`tp_dict` with the dictionary C-API.
+
.. c:member:: descrgetfunc PyTypeObject.tp_descr_get