There are at least three reasons, that I know of, to why you could be seeing mysql error 1005 (errno: 150)

• Adding a foreign key constraint when column types don't match
• Not enough privileges to execute the script
• Trying to delete an index that is needed by some other index or constraint ("error on rename of")

Use "SHOW ENGINE INNODB STATUS;" to view the error:

ALTER TABLE videos ADD constraint fk_videos_channels_id FOREIGN KEY (channel_id) REFERENCES channels (id);
SHOW ENGINE INNODB STATUS;