Language: Python
Label: vulnerable
CWE: CWE-203 \u2014 Observable Discrepancy
The product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor, which exposes security-relevant information about the state of the product, such as whether a particular operation was successful or not.

Code:
    def test_it_checks_for_existing_users(self):
        alice = User(username="alice", email="alice@example.org")
        alice.save()

        form = {"identity": "alice@example.org", "tz": ""}
        r = self.client.post("/accounts/signup/", form)
        self.assertContains(r, "already exists")