Extended explanation on how and are handled in

docs/topics/testing/overview.txt

@@ -925,14 +925,23 @@ to test the effects of commit and rollback:
 
 * A ``TestCase``, on the other hand, does not truncate tables after a test.
   Instead, it encloses the test code in a database transaction that is rolled
-  back at the end of the test.  It also prevents the code under test from
-  issuing any commit or rollback operations on the database, to ensure that the
-  rollback at the end of the test restores the database to its initial state.
+  back at the end of the test. Both explicit commits like ``transaction.commit()``
+  and implicit ones that may be caused by ``Model.save()`` are instead replaced
+  with a ``nop`` operation. It guarantees that the rollback at the end of the
+  test restores the database to its initial state.
 
   When running on a database that does not support rollback (e.g. MySQL with the
   MyISAM storage engine), ``TestCase`` falls back to initializing the database
   by truncating tables and reloading initial data.
 
+.. note::
+
+    While all ``commit`` and ``rollback`` operations still *appear* to work
+    when used in ``TestCase`` they do not cause any side effect, or more
+    precisely, no actuall commit or rollback will be performed by the database.
+    This can cause your test to pass or fail unexpectedly. Always use
+    ``TransactionalTestCase`` when testing transactional behavior.
+
 .. note::
 
     .. versionchanged:: 1.5