I tried dropping Hibernate 3.5.0 into a project that currently uses Hibernate 3.3.2, but I started running into an exception that I hadn't experienced before:

IllegalStateException: Cannot make an immutable entity modifiable.

As far as I can tell, this is happening when refresh is called on a detached object for a class (representing a database view) that is mapped with mutable="false". After loading the refreshed object in DefaultRefreshEventListener.onRefresh, Hibernate sets the object's read-only status to whatever the defaultReadOnly is for the session (which is false by default). That then conflicts with the fact the class is mapped as immutable, causing the IllegalStateException in EntityEntry.setReadOnly.

Is this expected behavior? If so, how should I adapt the code since this appears to have worked prior to 3.5.0?

Sounds truly like a real bug.
(I cannot imagine this as expected or intended behavior)

An update ... one of my colleagues created a test case and filed HHH-5207 in JIRA. Looks like it was confirmed as a bug and will be fixed in 3.5.2.