A poorly coded trigger on the referencing table has the ability to break foreign keys, and as a result create a database which cannot be dumped and reloaded.
The BEFORE DELETE trigger accidentally does RETURN NEW, which suppresses the DELETE action by the foreign key trigger. This allows the record from the referenced table to be deleted and the record in the referencing table to remain in place.
While I don't expect Pg to do what the coder meant, but it should throw an error and not leave foreign key'd data in an invalid state.
This applies to both 9.1 and 9.2.
Please see attached bug.sql.