A Rails 3.2.0 app, working fine with Thin web server, both locally and on Heroku cedar stack.
After:
$ git branch work
$ git checkout work
$ rails se
I has this error because i was running rails-dev-box with Rails inside of it.
Port 3000 in the host computer is forwarded to port 3000 in the virtual machine.
Thus, applications running in the virtual machine can be accessed via
localhost:3000 in the host computer.
So is logged out from Vagrant and shutted down it:
vagrant@rails-dev-box:/vagrant/rails$ exit
$ vagrant halt
That helped me.