diff --git a/changelog/13480.bugfix.rst b/changelog/13480.bugfix.rst
new file mode 100644
index 00000000000..72bc20b0d73
--- /dev/null
+++ b/changelog/13480.bugfix.rst
@@ -0,0 +1,3 @@
+Modified a few self-tests to explicitly pass ``-Werror``
+to ``runpytester()``, in order to fix test failures when the test suite
+is run with ``-Wdefault`` or a similar override.
diff --git a/testing/test_threadexception.py b/testing/test_threadexception.py
index 5dad07b8b85..f4595ec435d 100644
--- a/testing/test_threadexception.py
+++ b/testing/test_threadexception.py
@@ -211,7 +211,7 @@ def test_it(request):
         """
     )
 
-    result = pytester.runpytest()
+    result = pytester.runpytest("-Werror")
 
     # TODO: should be a test failure or error
     assert result.ret == pytest.ExitCode.INTERNAL_ERROR
diff --git a/testing/test_unraisableexception.py b/testing/test_unraisableexception.py
index 6c0dc542e93..15a8d47e1a2 100644
--- a/testing/test_unraisableexception.py
+++ b/testing/test_unraisableexception.py
@@ -257,7 +257,7 @@ def test_it():
     )
 
     with _disable_gc():
-        result = pytester.runpytest()
+        result = pytester.runpytest("-Werror")
 
     # TODO: should be a test failure or error
     assert result.ret == pytest.ExitCode.INTERNAL_ERROR
diff --git a/testing/test_warnings.py b/testing/test_warnings.py
index c302e7c6e3c..b1c64dc9332 100644
--- a/testing/test_warnings.py
+++ b/testing/test_warnings.py
@@ -149,6 +149,7 @@ def test_func(fix):
     )
 
 
+@pytest.mark.skip("issue #13485")
 def test_works_with_filterwarnings(pytester: Pytester) -> None:
     """Ensure our warnings capture does not mess with pre-installed filters (#2430)."""
     pytester.makepyfile(