First of all, this happened to me trying to migrate db with a rake task on the production server.
NOTE: please take a snapshot or a backup of your database data before starting with migrations.
Are you developing a Rails 3.0.x app? Are you using slim_scrooge gem?
Did your migration worked on development/test environment, but it looks having problem on production server?
Better: it looks like working but when you check your DB or schema.rb file you see nothing happened?