diff --git a/erpnext/controllers/tests/test_taxes_and_totals.py b/erpnext/controllers/tests/test_taxes_and_totals.py
new file mode 100644
index 00000000000..896417b43fe
--- /dev/null
+++ b/erpnext/controllers/tests/test_taxes_and_totals.py
@@ -0,0 +1,31 @@
+from unittest.mock import patch
+
+import frappe
+from frappe.tests import IntegrationTestCase
+
+from erpnext.controllers.taxes_and_totals import calculate_taxes_and_totals
+from erpnext.selling.doctype.sales_order.test_sales_order import make_sales_order
+
+
+class TestTaxesAndTotals(IntegrationTestCase):
+	def test_regional_round_off_accounts(self):
+		"""
+		Regional overrides cannot extend the list in-place — the return
+		value must be assigned back to frappe.flags.round_off_applicable_accounts.
+		"""
+		test_account = "_Test Round Off Account"
+
+		def mock_regional(company, account_list: list) -> list:
+			# Simulates a regional override
+			account_list.extend([test_account])
+			return account_list
+
+		so = make_sales_order(do_not_save=True)
+
+		with patch(
+			"erpnext.controllers.taxes_and_totals.get_regional_round_off_accounts",
+			mock_regional,
+		):
+			calculate_taxes_and_totals(so)
+
+		self.assertIn(test_account, frappe.flags.round_off_applicable_accounts)