I found out that the JSF 2.0 Flash scope is implemented in Mojarra 2.x via a cookie. What seems to happen is that when navigating from view A to view B using The Flash, JSF send
If there is resource, such as a file or, cookie... for sure, there will always be a chance to be a race condition. But you should investigate, how to avoid it.
I think the answer to have it perfectly save are EJB 3.0 We use EJB 3.0 because are transactional safe. In the same way we pass a request to an EJB to insert data in a database, you can interact with your flash scope using an EJB as a negociator. That way your transaction will be secured and that means you will be race conditon risk free.