diff --git a/Doc/c-api/iter.rst b/Doc/c-api/iter.rst
--- a/Doc/c-api/iter.rst
+++ b/Doc/c-api/iter.rst
@@ -14,6 +14,10 @@
 
    Return true if the object *o* supports the iterator protocol.
 
+   This function can return a false positive in the case of old-style
+   classes because those classes always define a :c:member:`tp_iternext`
+   slot with logic that either invokes a :meth:`next` method or raises
+   a :exc:`TypeError`.
 
 .. c:function:: PyObject* PyIter_Next(PyObject *o)