Demo site bug - suspect bug is just demo site not Civi - something to do with MYSQL

I tried to create a custom data group on Demo (attached to Contributions). When I hit save I got an error.

http://drupal.demo.civicrm.org/civicrm/admin/custom/group/field

Ref fields - Custom Fields

unrecoverable error
Sorry. A non-recoverable error has occurred.

DB Error: can not create

Database Error Code: Can't create table './civicrm_demo_drupal/#sql-4f67_44c9b7.frm' (errno: 150), 1005

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Error Details:

Array
(
[callback] => Array
(
[0] => CRM_Core_Error
[1] => handle
)

[code] => -15
[message] => DB Error: can not create
[mode] => 16
[debug_info] => ALTER TABLE civicrm_value_ref_fields_11
ADD COLUMN `contact_ref_19` int unsigned,
ADD INDEX INDEX_contact_ref_19 ( contact_ref_19 ),
ADD CONSTRAINT FK_civicrm_value_ref_fields_11_contact_ref_19 FOREIGN KEY ( `contact_ref_19` ) REFERENCES civicrm_contact_en_US ( id ) ON DELETE SET NULL [nativecode=1005 ** Can't create table './civicrm_demo_drupal/#sql-4f67_44c9b7.frm' (errno: 150)]
[type] => DB_Error
[user_info] => ALTER TABLE civicrm_value_ref_fields_11
ADD COLUMN `contact_ref_19` int unsigned,
ADD INDEX INDEX_contact_ref_19 ( contact_ref_19 ),
ADD CONSTRAINT FK_civicrm_value_ref_fields_11_contact_ref_19 FOREIGN KEY ( `contact_ref_19` ) REFERENCES civicrm_contact_en_US ( id ) ON DELETE SET NULL [nativecode=1005 ** Can't create table './civicrm_demo_drupal/#sql-4f67_44c9b7.frm' (errno: 150)]
[to_string] => [db_error: message="DB Error: can not create" code=-15 mode=callback callback=CRM_Core_Error::handle prefix="" info=" ALTER TABLE civicrm_value_ref_fields_11
ADD COLUMN `contact_ref_19` int unsigned,
ADD INDEX INDEX_contact_ref_19 ( contact_ref_19 ),
ADD CONSTRAINT FK_civicrm_value_ref_fields_11_contact_ref_19 FOREIGN KEY ( `contact_ref_19` ) REFERENCES civicrm_contact_en_US ( id ) ON DELETE SET NULL [nativecode=1005 ** Can't create table './civicrm_demo_drupal/#sql-4f67_44c9b7.frm' (errno: 150)]"]
)