title: Why does `rake db:seed` fail with "Don't know how to build task 'db:seed:original'"?
body: <p>When I try to seed my Rails 4.0 app, I get this error:</p>
  
  <pre class="lang-none prettyprint-override"><code>$ rake db:seed --trace
  ** Invoke db:seed (first_time)
  ** Invoke db:seed:common (first_time)
  rake aborted!
  Don't know how to build task 'db:seed:original'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/task_manager.rb:62:in `[]'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/task.rb:57:in `lookup_prerequisite'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/task.rb:53:in `block in prerequisite_tasks'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/task.rb:53:in `map'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/task.rb:53:in `prerequisite_tasks'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/task.rb:199:in `invoke_prerequisites'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/task.rb:178:in `block in invoke_with_call_chain'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/monitor.rb:211:in `mon_synchronize'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/task.rb:172:in `invoke_with_call_chain'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/task.rb:201:in `block in invoke_prerequisites'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/task.rb:199:in `each'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/task.rb:199:in `invoke_prerequisites'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/task.rb:178:in `block in invoke_with_call_chain'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/monitor.rb:211:in `mon_synchronize'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/task.rb:172:in `invoke_with_call_chain'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/task.rb:165:in `invoke'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/application.rb:150:in `invoke_task'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/application.rb:106:in `block (2 levels) in top_level'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/application.rb:106:in `each'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/application.rb:106:in `block in top_level'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/application.rb:115:in `run_with_threads'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/application.rb:100:in `top_level'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/application.rb:78:in `block in run'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/application.rb:176:in `standard_exception_handling'
  /opt/rbenv/versions/2.2.0/lib/ruby/2.2.0/rake/application.rb:75:in `run'
  /opt/rbenv/versions/2.2.0/bin/rake:33:in `&lt;main&gt;'
  Tasks: TOP =&gt; db:seed =&gt; db:seed:common
  </code></pre>
  
  <p>But I can't find any references to a seed task called <code>original</code>. Is this something built in to Rails?</p>
  
  <p>If I call any of my seed tasks manually by e.g. <code>rake db:seed:whatever</code>, it works fine.</p>
  
  <p>Amusingly, after I created a <strong>empty</strong> file called <code>original.seeds.rb</code> in my seeds folder, the seeding ran without error.</p>
  
  <p>What caused this error?</p>