During mapping initialization, if some class references an unmapped class (e.g. in a <set> or @OneToMany declaration where the targetEntity is not known to Hibernate), an exception is thrown:
org.hibernate.MappingException: Association references unmapped class: com.bar.Foo

Probably in most cases, a mapping is actually missing and adding it solves the issue.
However, there may be the case in which you want to remove or modify the mapping containing the association to the unmapped class. Unfortunately there is no information in the stack trace leading to this mapping.
It would be helpful, if the name of the referencing entity and table were part of the message.

These informations are present at the time the exception is thrown which happens in org.hibernate.cfg.HbmBinder.bindCollectionSecondPass (line 2577 in version 4.3.8).
The line
throw new MappingException( "Association references unmapped class: " + assocClass );
could be refactored to something like
throw new MappingException(String.format("Association %s for table %s references unmapped class: %s", oneToMany.getReferencedEntityName(), oneToMany.getTable().getName(), assocClass));