My favourite is a logout button with a logout API that fails. (Not a huge pratical concern, I admit, because it's a local attack.) Nobody ever notices because it still shows the logout screen, which hides the API error toast (if errors were even displayed). The still valid refresh token stays in sessionStorage (or even localStorage) while the app displays "logged out". (Bonus points if you cleared the access token in the error handler but not the refresh token, and on page reload you ask the user to log in again despite having a valid token.)

Or a login form that gets hidden after login, but clears the username and password only when you click "login back in". (Bonus points if the backend also enforces a 5min session timeout "for security".)