MySQL InnoDB Foreign Key Error

I was getting Error 1005 "can't create table" ... errno 150 when trying to create a foreign key using the following

ALTER TABLE `user`
  ADD CONSTRAINT FOREIGN KEY (`group_id`) REFERENCES `group` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION;

What I needed to do to fix it was to create an index on group_id in the user table. Also if you get error 1452 make sure you don't already have fields filled which would otherwise be invalid.

Delete a Foreign Key

Get the Key Name from the bottom of an export, then

ALTER TABLE page_titles DROP FOREIGN KEY 0_172