ALTER TABLE course add constraint
fk_lang
FOREIGN KEY (lang_id) references content_language(lang_id);

ALTER TABLE book_author add constraint
fk_lang
FOREIGN KEY (author_id) references author(author_id);



fails with:

ERROR 1005 (HY000): Can't create table 'catalogue.#sql-5bb4_1b2' (errno: 121)



perror 121:

OS error code 121:  Remote I/O error
MySQL error code 121: Duplicate key on write or update