diff --git a/Lib/test/test_set.py b/Lib/test/test_set.py
index c0df9507bd7f5e..6e7718253e4def 100644
--- a/Lib/test/test_set.py
+++ b/Lib/test/test_set.py
@@ -675,6 +675,35 @@ def __hash__(self):
         with self.assertRaises(KeyError):
             myset.discard(elem2)
 
+    def test_set_add_to_dummy_slot(self):
+        # gh-141805
+        tasks = set()
+
+        class Dummy:
+            def __hash__(self):
+                return 0
+
+        class CorruptTrigger:
+            triggered = False
+
+            def __hash__(self):
+                return 0
+
+            def __eq__(self, value):
+                if not self.triggered:
+                    self.triggered = True
+                    tasks.add(self)
+                return False
+
+        tasks.add(Dummy())
+        tasks.add(Dummy())
+        tasks.pop()
+        self.assertEqual(len(tasks), 1)
+        tasks.add(CorruptTrigger())
+        self.assertEqual(len(tasks), 2)
+        # cover the case in gh-141805 that triggers segfault
+        repr(tasks)
+
 
 class SetSubclass(set):
     pass
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-11-27-17-47-09.gh-issue-141805.TT0biF.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-11-27-17-47-09.gh-issue-141805.TT0biF.rst
new file mode 100644
index 00000000000000..37ec77437bbefe
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-11-27-17-47-09.gh-issue-141805.TT0biF.rst
@@ -0,0 +1,2 @@
+Fix set corruption due to invalid length calculation when set additions occur
+in the element comparison (:meth:`object.__eq__`) methods.
diff --git a/Objects/setobject.c b/Objects/setobject.c
index 5f868c858273ee..b88e5696965575 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -298,6 +298,7 @@ set_add_entry_takeref(PySetObject *so, PyObject *key, Py_hash_t hash)
             else if (entry->hash == -1) {
                 assert (entry->key == dummy);
                 freeslot = entry;
+                goto found_unused_or_dummy;
             }
             entry++;
         } while (probes--);