diff --git a/bin/mechanicaldiff.py b/bin/mechanicaldiff.py
index 453f04f..f583fc8 100755
--- a/bin/mechanicaldiff.py
+++ b/bin/mechanicaldiff.py
@@ -88,6 +88,7 @@ def main() -> None:
 
     output_lines = []
     output_lines.extend(preamble_lines)
+    left_out = False
 
     for section in sections:
         if not section["hunks"]:
@@ -99,6 +100,8 @@ def main() -> None:
             hunk_text = "".join(hunk_lines)
             if should_include_hunk(hunk_text, search, replace):
                 kept_hunks.append(hunk_lines)
+            else:
+                left_out = True
 
         if not kept_hunks:
             continue
@@ -108,6 +111,8 @@ def main() -> None:
             output_lines.extend(hunk_lines)
 
     sys.stdout.write("".join(output_lines))
+    if left_out:
+        raise SystemExit(1)
 
 
 if __name__ == "__main__":
diff --git a/test/mechanicaldiff.bats b/test/mechanicaldiff.bats
index 8f26fa4..46f779d 100755
--- a/test/mechanicaldiff.bats
+++ b/test/mechanicaldiff.bats
@@ -26,6 +26,6 @@ setup() {
   git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
     >"$BATS_TEST_TMPDIR/diff" || true
   run mechanicaldiff.py foo bar <"$BATS_TEST_TMPDIR/diff"
-  assert_success
+  assert_failure
   assert_output ""
 }