If you've force-closed your Rails server instance for whatever reason (like a bug that froze up the server), you might run into issues when you try and restart it. For example, you might get the error "WARN TCPServer Error: Address already in use - bind(2) Exiting" What do you do?
Assuming that you're running on port 3000, find the PID of the process with
lsof -wni tcp:3000
and kill the process with
kill -9 PID