diff --git a/tests/correlators_test.py b/tests/correlators_test.py
index cd629275..9df114d6 100644
--- a/tests/correlators_test.py
+++ b/tests/correlators_test.py
@@ -804,3 +804,18 @@ def test_prune_with_Nones():
 
     for t in range(T):
         assert np.all(pruned_then_padded.content[t] == padded_then_pruned.content[t])
+
+
+def test_Corr_padding_default_not_shared():
+    data = [pe.pseudo_Obs(i + 1, 0.1, "e") for i in range(5)]
+    c1 = pe.Corr(data)
+    c2 = pe.Corr(data)
+    assert c1 is not c2
+    assert len(c1.content) == len(c2.content) == 5
+    assert all(a[0] == b[0] for a, b in zip(c1.content, c2.content))
+
+
+def test_Corr_unhashable():
+    c = pe.Corr([pe.pseudo_Obs(i + 1, 0.1, "e") for i in range(5)])
+    with pytest.raises(TypeError):
+        hash(c)
diff --git a/tests/integrate_test.py b/tests/integrate_test.py
index 07a4313b..5bfe1260 100644
--- a/tests/integrate_test.py
+++ b/tests/integrate_test.py
@@ -49,3 +49,24 @@ def test_integration():
     assert r1 == -r2
     iamzero, _ = pe.integrate.quad(F, pobs, 1, 1)
     assert iamzero == 0
+
+
+def test_integrate_per_parameter_derivatives():
+    # \int_0^1 p0*x + p1*x^2 dx = p0/2 + p1/3
+    # If the lambda closure in integrate.quad failed to bind `i` per-iteration,
+    # all per-parameter derivatives would collapse to a single value.
+    def f(p, x):
+        return p[0] * x + p[1] * x ** 2
+
+    p = [pe.cov_Obs(1.0, 0.1 ** 2, "p0"), pe.cov_Obs(2.0, 0.2 ** 2, "p1")]
+    res, _ = pe.integrate.quad(f, p, 0.0, 1.0)
+    res.gm()
+
+    ana_val = p[0].value / 2 + p[1].value / 3
+    assert np.isclose(res.value, ana_val)
+
+    grad0 = res.covobs["p0"].grad.item()
+    grad1 = res.covobs["p1"].grad.item()
+    assert np.isclose(grad0, 0.5)
+    assert np.isclose(grad1, 1.0 / 3.0)
+    assert not np.isclose(grad0, grad1)
diff --git a/tests/misc_test.py b/tests/misc_test.py
index 99bb5056..028ea6da 100644
--- a/tests/misc_test.py
+++ b/tests/misc_test.py
@@ -22,3 +22,18 @@ def test_obs_errorbar():
 
 def test_print_config():
     pe.print_config()
+
+
+def test_pseudo_Obs_seed_independence():
+    # pseudo_Obs now uses a module-local np.random.default_rng() generator,
+    # so np.random.seed() no longer controls its output. The per-sample
+    # deltas therefore differ between successive calls even with a re-seed,
+    # though the normalized value / dvalue still match the requested inputs.
+    np.random.seed(0)
+    a = pe.pseudo_Obs(1.0, 0.1, "e")
+    np.random.seed(0)
+    b = pe.pseudo_Obs(1.0, 0.1, "e")
+
+    assert not np.allclose(a.deltas["e"], b.deltas["e"])
+    assert np.isclose(a.value, b.value)
+    assert np.isclose(a.dvalue, b.dvalue)
diff --git a/tests/obs_test.py b/tests/obs_test.py
index a6cee885..3a9eb0a8 100644
--- a/tests/obs_test.py
+++ b/tests/obs_test.py
@@ -1211,6 +1211,33 @@ def test_hash():
     assert hash(o1) != hash(o2)
 
 
+def test_Obs_mismatched_lengths():
+    with pytest.raises(ValueError):
+        pe.Obs([np.random.rand(100), np.random.rand(100)], ["a"])
+    with pytest.raises(ValueError):
+        pe.Obs([np.random.rand(100)], ["a"], idl=[range(100), range(50)])
+    with pytest.raises(ValueError):
+        pe.Obs([np.random.rand(100), np.random.rand(100)], ["a", "b"],
+               idl=[range(100)])
+
+
+def test_details_fractional_Nsigma(capsys):
+    # N_sigma only appears in details() output when tau_exp > 0.
+    # Verifies the format change from '%1.0i' (integer truncation) to
+    # f'{:g}' preserves fractional digits.
+    o = pe.pseudo_Obs(1.0, 0.1, "e")
+    o.gamma_method(tau_exp=1.5, N_sigma=1.5)
+    o.details()
+    out = capsys.readouterr().out
+    assert "N_\N{GREEK SMALL LETTER SIGMA}=1.5" in out
+
+
+def test_CObs_unhashable():
+    c = pe.CObs(pe.pseudo_Obs(1.0, 0.1, "e"), pe.pseudo_Obs(0.0, 0.1, "e"))
+    with pytest.raises(TypeError):
+        hash(c)
+
+
 def test_gm_alias():
     samples = np.random.rand(500)