I'm not sure if I should include an unit test. WIP patch for that:

diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 350ef77163..9c0d0cf41a 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -4718,6 +4718,18 @@ negative_refcount(PyObject *self, PyObject *Py_UNUSED(args))
 #endif
 
 
+static PyObject *
+corrupted_object(PyObject *self, PyObject *Py_UNUSED(args))
+{
+    PyObject *obj = PyList_New(0);
+    if (obj == NULL) {
+        return NULL;
+    }
+    obj->ob_type = NULL;
+    return obj;
+}
+
+
 static PyMethodDef TestMethods[] = {
     {"raise_exception",         raise_exception,                 METH_VARARGS},
     {"raise_memoryerror",       raise_memoryerror,               METH_NOARGS},
@@ -4948,6 +4960,7 @@ static PyMethodDef TestMethods[] = {
 #ifdef Py_REF_DEBUG
     {"negative_refcount", negative_refcount, METH_NOARGS},
 #endif
+    {"corrupted_object", corrupted_object, METH_NOARGS},
     {NULL, NULL} /* sentinel */
 };
 


Tested manually using this script:
---
import gc, _testcapi, sys

gc.enable_object_debugger(1)
x = _testcapi.corrupted_object()
y = []
y = None
# Debugger should trigger here
x = None
---