ruby-debug(19) is pretty famous as an important, but unmaintained package that tend to cause a lot of problems. There are hundreds of solutions around the web how to handle it. The simplest solution, however, is to use a neat fork of ruby-debug(19), simply called debugger which incorporates various fixes and works with Ruby 1.9.2 and 1.9.3.
To start using it, simply put the following line into your Gemfile, instead of ruby-debug(19)
gem 'debugger`or just require it in your code and then start it
require 'debugger'
debugger