Hi Thiago, If the entity cliente is managed by another instance of an entity manager manager you will see this exception. Make sure you either close (or clear, if you reuse the entity manager) any other entity managers that may have managed this entity. Closing the entity manager will put cliente in a detached state, allowing you to merge it into a new persistence context.
I don't know exactly how faces is implemented, but from the stack trace it appears to be using a servlet. Servlets are typically a singleton, so you could be hitting a concurrency issue, where multiple clients are calling persistir() and using the cliente entity at the same time. As a simple test, you could synchronize persistir() to see if that solves the problem. Although, I wouldn't recommend that as a long term solution since it may cause a severe performance problem. -Jeremy
