mysql Foreign key constraint is incorrectly formed error

For anyone facing this problem, just run SHOW ENGINE INNODB STATUS and see the LATEST FOREIGN KEY ERROR section for details.

I ran into this same problem with HeidiSQL. The error you receive is very cryptic. My problem ended up being that the foreign key column and the referencing column were not of the same type or length.

The foreign key column was SMALLINT(5) UNSIGNED and the referenced column was INT(10) UNSIGNED. Once I made them both the same exact type, the foreign key creation worked perfectly.