Hibernate: How to fix “identifier of an instance altered from X to Y”?

Question

org.hibernate.HibernateException: identifier of an instance 
of org.cometd.hibernate.User altered from 12 to 3

in fact, my user table

Answer 1

In my case, a template had a typo so instead of checking for equivalency (==) it was using an assignment equals (=).

So I changed the template logic from:

if (user1.id = user2.id) ...

to

if (user1.id == user2.id) ...

and now everything is fine. So, check your views as well!