 <p>Have you tried</p> <pre><code>if foo.bar == "crazy value" require 'ruby-debug'; debugger end </code></pre> <p>This should place a breakpoint that triggers when you run <code>bundle exec rails server</code> normally, and only when <code>foo.bar</code> has the value you (don't) want.</p> <p>If you are using bundler, and Ruby 1.9 make sure you have </p> <pre><code>gem 'ruby-debug19', :require =&gt; 'ruby-debug' </code></pre> <p>in your Gemfile (in the development and test group).</p>