diff --git a/src/git_commit_guard/__init__.py b/src/git_commit_guard/__init__.py
index 801a218..df7b7f2 100644
--- a/src/git_commit_guard/__init__.py
+++ b/src/git_commit_guard/__init__.py
@@ -151,6 +151,12 @@ def _download_if_missing(resource):
         nltk.download(resource.rsplit("/", maxsplit=1)[-1], quiet=True)
 
 
+def _format_allowed_hint(allowed, kind):
+    if len(allowed) <= len(TYPES):
+        return f"(allowed: {', '.join(sorted(allowed))})"
+    return f"(see configured {kind})"
+
+
 def _strip_comments(message):
     return "\n".join(
         line for line in message.split("\n") if not line.lstrip().startswith("#")
@@ -178,13 +184,16 @@ def check_subject(  # noqa: PLR0913 Too many arguments in function definition (9
         return None
 
     if m.group("type") not in allowed_types:
-        result.error(f"unknown type: {m.group('type')}", check=Check.SUBJECT)
+        bad_type = m.group("type")
+        hint = _format_allowed_hint(allowed_types, "types")
+        result.error(f"unknown type: {bad_type} {hint}", check=Check.SUBJECT)
 
     scope = m.group("scope")
     if require_scope and scope is None:
         result.error("scope is required", check=Check.SUBJECT)
     if allowed_scopes and scope is not None and scope not in allowed_scopes:
-        result.error(f"unknown scope: {scope}", check=Check.SUBJECT)
+        hint = _format_allowed_hint(allowed_scopes, "scopes")
+        result.error(f"unknown scope: {scope} {hint}", check=Check.SUBJECT)
 
     desc = m.group("desc")
     if require_lowercase and desc[0].isupper():
@@ -249,7 +258,10 @@ def check_body(lines, result):
 
 def check_signed_off(message, result):
     if not SIGNED_OFF_RE.search(message):
-        result.error("missing 'Signed-off-by' trailer", check=Check.SIGNED_OFF)
+        result.error(
+            "missing 'Signed-off-by' trailer — use 'git commit -s'",
+            check=Check.SIGNED_OFF,
+        )
 
 
 def check_subject_pattern(subject, pattern, result):
diff --git a/tests/test_git_commit_guard.py b/tests/test_git_commit_guard.py
index f3d3517..d58046f 100644
--- a/tests/test_git_commit_guard.py
+++ b/tests/test_git_commit_guard.py
@@ -124,6 +124,25 @@ def test_unknown_type(self):
         check_subject("unknown: add thing", r)
         assert not r.ok
 
+    def test_unknown_type_with_default_lists_all_allowed(self):
+        r = Result()
+        check_subject("unknown: add thing", r)
+        assert any(
+            "allowed: " in m and "feat" in m and "fix" in m for _, _, m in r.errors
+        )
+
+    def test_unknown_type_with_smaller_set_lists_them(self):
+        r = Result()
+        check_subject("foo: add thing", r, allowed_types=frozenset({"feat", "fix"}))
+        assert any("allowed: feat, fix" in m for _, _, m in r.errors)
+
+    def test_unknown_type_with_larger_than_default_points_at_config(self):
+        r = Result()
+        oversized = frozenset({f"t{i}" for i in range(20)})
+        check_subject("foo: add thing", r, allowed_types=oversized)
+        assert any("see configured types" in m for _, _, m in r.errors)
+        assert not any("allowed:" in m for _, _, m in r.errors)
+
     def test_uppercase_description(self):
         r = Result()
         check_subject("fix: Add token", r)
@@ -184,6 +203,22 @@ def test_scope_not_in_allowlist_fails(self):
         check_subject("fix(api): add token", r, allowed_scopes=frozenset(["auth"]))
         assert not r.ok
 
+    def test_unknown_scope_with_small_set_lists_them(self):
+        r = Result()
+        check_subject(
+            "fix(api): add token",
+            r,
+            allowed_scopes=frozenset({"auth", "db"}),
+        )
+        assert any("allowed: auth, db" in m for _, _, m in r.errors)
+
+    def test_unknown_scope_with_larger_than_default_points_at_config(self):
+        r = Result()
+        oversized = frozenset({f"s{i}" for i in range(20)})
+        check_subject("fix(foo): add token", r, allowed_scopes=oversized)
+        assert any("see configured scopes" in m for _, _, m in r.errors)
+        assert not any("allowed:" in m for _, _, m in r.errors)
+
     def test_no_scope_with_allowlist_passes(self):
         r = Result()
         check_subject("fix: add token", r, allowed_scopes=frozenset(["auth"]))
@@ -325,6 +360,11 @@ def test_missing(self):
         check_signed_off("fix: add thing\n\nbody", r)
         assert not r.ok
 
+    def test_missing_message_hints_at_git_commit_dash_s(self):
+        r = Result()
+        check_signed_off("fix: add thing\n\nbody", r)
+        assert any("git commit -s" in m for _, _, m in r.errors)
+
     def test_malformed_no_email(self):
         r = Result()
         check_signed_off("fix: add thing\n\nSigned-off-by: John Doe", r)