From my post on the dev list:

We just hit the ClassCastException problem with 2nd level cache
reported by other people.

But... we're using 4.3.4.Final so I'm not sure this issue has really
been fixed with 4.3.4.Final.

Typically, we have the following hierarchy:

AbstractUser
    TechnicalUser (id = 1)
    AccountManager (id = 2)

entityManager.find(AccountManager.class, 1) returns the TechnicalUser
1 and not null if the 2nd level cache is enabled (and the object is in
the cache).

I'm pretty sure it's not the expected behavior.