If you catch errno: 150 “Foreign key constraint is incorrectly formed” when you try to create a new table in mySQL(or another relational DB) then you can use a simple way to find a bad foreign key(s). Just execute this query:
SELECT CONSTRAINT_NAME, TABLE_NAME, COLUMN_NAME, REFERENCED_TABLE_NAME, REFERENCED_COLUMN_NAME FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE WHERE REFERENCED_TABLE_NAME = '<your table name here>' OR TABLE_NAME = '<your table name here>';
After that you can see where this bad table using and you can go and delete bad foreign key(s).
Happy SQLing)