RSpec & Capybara: How to fix undefined method `feature' for main:Object (NoMethodError)

While working on the Ohana API during the Code for America fellowship, I came across a strange situation. My feature specs didn't include require "spec_helper" at the top of each file, yet I was able to run all of the specs like so:

$ rspec

Or just all of the feature specs:

$ rspec spec/features

And individual ones as well:

$ rspec spec/features/signin_spec.rb

Then when I cloned the repo to a new directory (to test the installation instructions in the README), I noticed that I could no longer run specific tests. If I tried to run rspec spec/features/signin_spec.rb for example, I would get this error:

/Users/monfresh/Desktop/ohana-api/spec/features/signin_spec.rb:1:in `<top (required)>': undefined method `feature' for main:Object (NoMethodError)
  from /Users/monfresh/.rvm/gems/ruby-2.0.0-p353@ohana-api/gems/rspec-core-2.14.5/lib/rspec/core/configuration.rb:896:in `load'
  from /Users/monfresh/.rvm/gems/ruby-2.0.0-p353@ohana-api/gems/rspec-core-2.14.5/lib/rspec/core/configuration.rb:896:in `block in load_spec_files'
  from /Users/monfresh/.rvm/gems/ruby-2.0.0-p353@ohana-api/gems/rspec-core-2.14.5/lib/rspec/core/configuration.rb:896:in `each'
  from /Users/monfresh/.rvm/gems/ruby-2.0.0-p353@ohana-api/gems/rspec-core-2.14.5/lib/rspec/core/configuration.rb:896:in `load_spec_files'
  from /Users/monfresh/.rvm/gems/ruby-2.0.0-p353@ohana-api/gems/rspec-core-2.14.5/lib/rspec/core/command_line.rb:22:in `run'
  from /Users/monfresh/.rvm/gems/ruby-2.0.0-p353@ohana-api/gems/rspec-core-2.14.5/lib/rspec/core/runner.rb:80:in `run'
  from /Users/monfresh/.rvm/gems/ruby-2.0.0-p353@ohana-api/gems/rspec-core-2.14.5/lib/rspec/core/runner.rb:17:in `block in autorun'

The solution was to add require "spec_helper" to the top of each feature spec, but what I still haven't been able to figure out, is why I was able to get by for months without that require statement. If you know why, please send me a Tweet to enlighten me!