diff --git a/swh/core/db/common.py b/swh/core/db/common.py
--- a/swh/core/db/common.py
+++ b/swh/core/db/common.py
@@ -3,8 +3,9 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
-import inspect
 import functools
+import inspect
+import sys
 
 
 def remove_kwargs(names):
@@ -33,6 +34,21 @@
     return old_options
 
 
+def check_no_transaction(f_name):
+    """Checks there is no open DB transaction in the stack
+    (db + cur variables).
+    While it is not necessarily broken code, it is very likely
+    a mistake."""
+    frame = sys._getframe().f_back.f_back
+    while frame:
+        if "db" in frame.f_locals and "cur" in frame.f_locals:
+            raise AssertionError(
+                f'Calling function {f_name} without "db" and "cur" arguments '
+                "from a function ({frame.f_code.co_name}) with these variables."
+            )
+        frame = frame.f_back
+
+
 def db_transaction(**client_options):
     """decorator to execute Backend methods within DB transactions
 
@@ -55,6 +71,7 @@
                 apply_options(cur, old_options)
                 return ret
             else:
+                check_no_transaction(meth.__name__)
                 db = self.get_db()
                 try:
                     with db.transaction() as cur:
@@ -90,6 +107,7 @@
                 yield from meth(self, *args, **kwargs)
                 apply_options(cur, old_options)
             else:
+                check_no_transaction(meth.__name__)
                 db = self.get_db()
                 try:
                     with db.transaction() as cur:
diff --git a/swh/core/db/tests/test_db.py b/swh/core/db/tests/test_db.py
--- a/swh/core/db/tests/test_db.py
+++ b/swh/core/db/tests/test_db.py
@@ -170,6 +170,47 @@
     assert actual_sig == expected_sig
 
 
+def test_db_transaction_nested(mocker):
+    expected_cur = object()
+
+    called = False
+
+    class Storage:
+        @db_transaction()
+        def endpoint_bad_nesting(self, cur=None, db=None):
+            self.inner_endpoint()
+
+        @db_transaction()
+        def endpoint_good_nesting(self, cur=None, db=None):
+            self.inner_endpoint(cur=cur, db=db)
+
+        @db_transaction()
+        def inner_endpoint(self, cur=None, db=None):
+            nonlocal called
+            called = True
+
+    storage = Storage()
+
+    # 'with storage.get_db().transaction() as cur:' should cause
+    # 'cur' to be 'expected_cur'
+    db_mock = Mock()
+    db_mock.transaction.return_value = MagicMock()
+    db_mock.transaction.return_value.__enter__.return_value = expected_cur
+    mocker.patch.object(storage, "get_db", return_value=db_mock, create=True)
+
+    put_db_mock = mocker.patch.object(storage, "put_db", create=True)
+
+    with pytest.raises(AssertionError, match="Calling function inner_endpoint"):
+        storage.endpoint_bad_nesting()
+    assert not called
+    put_db_mock.assert_called_once_with(db_mock)
+    put_db_mock.reset_mock()
+
+    storage.endpoint_good_nesting()
+    assert called
+    put_db_mock.assert_called_once_with(db_mock)
+
+
 def test_db_transaction_generator(mocker):
     expected_cur = object()