Look the documentation for flush:

void org.hibernate.Session.flush()
Force the Session to flush. Must be called at the end of a unit of work, before commiting the transaction and closing the session (Transaction.commit() calls this method). Flushing is the process of synchronising the underlying persistent store with persistable state held in memory.

Why should I call the flush before commit the transaction? Is there any problem with this:

Transaction t = session.beginTransaction();
session.saveOrUpdate(object);
t.commit();
session.flush()

?

no problem, but flush after commit do nothing

flush send command to database and commit do it explicit

So, if you flush after the commit, the database won't be updated?

you may want to read parts of this ... http://forum.hibernate.org/viewtopic.php?t=950347

no, you needn't flush after commit - it do nothing
commit flush explicit and update and commit changes to database

I'm using FlushMode.NEVER.

What the answer for my first question considering the FlushMode.NEVER ?

you will have read only session.

In this case, is there some difference calling the flush before or after the commit ?

No.