You might have been happily running migrations on your development box for weeks and go to get the application set up on a production server.
And all might even have been going well until you ran

rake db:migrate

only to see it crash and burn like this.

** Invoke db:migrate (first_time)
** Invoke environment (first_time)
** Execute environment
rake aborted!
development database is not configured

Fear not ye intrepid developer!
This happens when you leave out the development specifications in your database.yml.
(If you don’t define development.rb, you’ll get a weirder “No such file or directory – …../development.rb” error)

Apparently rake db:migrate runs in development mode no matter what environment you’re in (still looking into the reason for this).
So what you have to do is call migrate like this

rake db:migrate RAILS_ENV=production <—- or whatever your environment is

That should get you back on the path to true happiness.