I've ran into this one with WildFly 9.0.1.Final and hibernate-core 4.2.20.Final a few weeks ago.
The mapping exception code is still exactly the same on master:
https://github.com/hibernate/hibernate-orm/blob/master/hibernate-core/src/main/java/org/hibernate/mapping/PersistentClass.java#L531
as it was on 4.2.20.Final, so this must affect hibernate 5.0 too.

@Steve I can't change the "Affects version", because I am not the original reporter...