diff --git a/B06_Section/B06_Section_Router_DesignSave.py b/B06_Section/B06_Section_Router_DesignSave.py
index d99f91da7..3abdb1ca2 100644
--- a/B06_Section/B06_Section_Router_DesignSave.py
+++ b/B06_Section/B06_Section_Router_DesignSave.py
@@ -15,6 +15,7 @@ from __future__ import annotations
 
 import json
 import logging
+import os
 from pathlib import Path
 from typing import Any
 from uuid import UUID
@@ -23,7 +24,6 @@ from fastapi import APIRouter, Depends
 from fastapi.responses import JSONResponse
 from pydantic import BaseModel, ConfigDict
 
-from common_util.common_util_auth import get_optional_session
 from B03_FileInput.B03_FileInput_Repository import get_project_storage_relative_path
 from B05_Profile.B05_Profile_Schema import (
     PageSettingsRequest,
@@ -34,6 +34,7 @@ from B05_Profile.B05_Profile_Structures_Schema import StructureSaveRequest
 from B06_Section.B06_Section_Router_Stations import PlusStationsRequest
 from B06_Section.B06_Section_Schema import SectionConfirmRequest
 from common_util import common_util_save_guard as guard
+from common_util.common_util_auth import get_optional_session
 from common_util.common_util_storage import resolve_stored_project_path
 from config.config_db import get_db_pool
 
@@ -113,6 +114,9 @@ async def _run_steps(
         except Exception as error:
             logger.exception("한 본문 저장 %s 차례 실패: project_id=%s", name, project_id)
             raise StepFailed(name, str(error) or type(error).__name__) from None
+        # 시험 갈래 — 화면에서 실패 · 되돌림을 돌려 볼 때만(서버를 이 환경 변수로 켬 · 평소 없음)
+        if os.environ.get("AISLO_DESIGN_SAVE_FAIL_AFTER") == name:
+            raise StepFailed(name, "시험 갈래 — 이 차례 뒤 일부러 실패")
         return out[name]
 
     if request.settings is not None:
diff --git a/resources/tester/test_472_6_design_save.py b/resources/tester/test_472_6_design_save.py
index 1b82a0dd5..5f1af1c1f 100644
--- a/resources/tester/test_472_6_design_save.py
+++ b/resources/tester/test_472_6_design_save.py
@@ -243,3 +243,12 @@ def test_같은_프로젝트_저장이_겹치면_뒤_요청이_기다림() -> No
     response = asyncio.run(run())
     body = json.loads(bytes(response.body).decode("utf-8"))
     assert body["status"] == "success" and body["waited"] is True and body["steps"] == {}
+
+
+@live
+def test_시험_갈래_환경_변수면_그_차례_뒤_실패_되돌림(monkeypatch) -> None:
+    before = _state(ROOT_DIR)
+    monkeypatch.setenv("AISLO_DESIGN_SAVE_FAIL_AFTER", "settings")
+    status, body = _save(_request())
+    assert status == 409 and body["step"] == "settings" and body["restored"] is True
+    assert _state(ROOT_DIR) == before