When importing biblio records (i.e via a migration script ), the biblionumber field (001 in unimarc) is inserted at the end of marcxml because C4::Biblio::ModBiblioMarc use insert_grouped_fields instead of insert_fields_ordered. This causes an error when we want to show a record which is very long (Serials with lot of items for example).
I was wrong. This bug has been fixed for a long time