diff --git a/tests/unit/test_frequency_detection.py b/tests/unit/test_frequency_detection.py
index ba4fba0b..672c7d3c 100644
--- a/tests/unit/test_frequency_detection.py
+++ b/tests/unit/test_frequency_detection.py
@@ -881,6 +881,30 @@ def test_accepts_a_single_path_as_a_string(self):
 
         assert detected == pd.Timedelta(days=31)
 
+    def test_warns_when_a_readable_file_has_no_detectable_frequency(self):
+        """A file that opens fine but carries a single timestep.
+
+        This is the other arm of ``if freq is not None`` -- distinct from a
+        file that fails to open, and the one that produces the "Could not
+        detect frequency for file" warning rather than "Error processing".
+        """
+        with tempfile.TemporaryDirectory() as tmpdir:
+            good = Path(tmpdir) / "daily.nc"
+            self._write(good, [0.0, 1.0, 2.0])
+            single = Path(tmpdir) / "one_step.nc"
+            self._write(single, [0.0])
+
+            with warnings.catch_warnings(record=True) as caught:
+                warnings.simplefilter("always")
+                detected = _detect_frequency_from_individual_files(
+                    [str(good), str(single)]
+                )
+
+            assert detected == pd.Timedelta(days=1)
+            messages = [str(w.message) for w in caught]
+            assert any("Could not detect frequency for file" in m for m in messages)
+            assert not any("Error processing file" in m for m in messages)
+
     def test_raises_when_no_file_yields_a_frequency(self):
         with tempfile.TemporaryDirectory() as tmpdir:
             path = Path(tmpdir) / "not_netcdf.nc"
diff --git a/tests/unit/test_qc_plots.py b/tests/unit/test_qc_plots.py
index 1e64c2ec..71593db7 100644
--- a/tests/unit/test_qc_plots.py
+++ b/tests/unit/test_qc_plots.py
@@ -281,6 +281,37 @@ def _fake(path, *, qc_dir=None, comparison_store=None, preferred_member=None):
 
         assert called_with == [nc_path]
 
+    def test_combined_overlay_loaded_when_comparison_store_given(self, temp_dir):
+        """The split-files path forwards its own open dataset to _load_overlay.
+
+        `generate_qc_plots` has an equivalent branch that is covered; this one
+        was not, so the `comparison_store` arm of
+        `generate_qc_plots_for_split_files` had never been executed.
+        """
+        paths = _write_split_cmip_files(temp_dir, "tas_Amon", n_chunks=2)
+        qc_dir = temp_dir / "qc"
+        from access_moppy.qc import plots as plots_module
+
+        with patch.object(
+            plots_module, "_load_overlay", return_value=None
+        ) as mock_overlay:
+            result = plots_module.generate_qc_plots_for_split_files(
+                paths,
+                qc_dir=qc_dir,
+                comparison_store=temp_dir / "comparison",
+                preferred_member="r1i1p1f1",
+            )
+
+        assert result == qc_dir
+        assert (qc_dir / "tas_Amon_timeseries.png").exists()
+        mock_overlay.assert_called_once()
+        kwargs = mock_overlay.call_args.kwargs
+        assert kwargs["var_name"] == "tas"
+        assert kwargs["store_path"] == temp_dir / "comparison"
+        assert kwargs["preferred_member"] == "r1i1p1f1"
+        # the dataset handed to the overlay is the combined one, not a single split
+        assert kwargs["ds"].sizes["time"] == 8
+
     def test_combined_timeseries_written(self, temp_dir):
         paths = _write_split_cmip_files(temp_dir, "tas_Amon", n_chunks=2)
         qc_dir = temp_dir / "qc"