fails.rb

module Example
  def to_yaml
    already_tested
  end
end

class ExampleHolder
  include Example
end

describe "Example" do
  subject { ExampleHolder.new }

  it "should write to a given file" do
    subject.should_receive(:already_tested).and_return({ 'foo' => 'bar', 'baz' => 'bom' })
    subject.to_yaml
  end
end

# $ spec spec/example_spec.rb
# F
#
# Failures:
#
#   1) Example should write to a given file
#      Failure/Error: already_tested
#      NoMethodError:
#        undefined method `already_tested' for #<ExampleHolder:0x9128c80>
#      # ./spec/example_spec.rb:3:in `to_yaml'
#      # ./spec/example_spec.rb:16:in `block (2 levels) in <top (required)>'
#
# Finished in 0.00098 seconds
# 1 example, 1 failure

passes.rb

module Example
  def needs_testing
    already_tested
  end
end

class ExampleHolder
  include Example
end

describe "Example" do
  subject { ExampleHolder.new }

  it "should write to a given file" do
    subject.should_receive(:already_tested).and_return({ 'foo' => 'bar', 'baz' => 'bom' })
    subject.needs_testing
  end
end

# $ spec spec/example_spec.rb
# .
#
# Finished in 0.00075 seconds
# 1 example, 0 failures

Note that the only difference is the NAME OF THE METHOD BEING TESTED. lolwut?

Strange additional info: If I add p subject.methods.sort - Object.methods in the middle of the example, the fail case prints [:already_tested] and the pass case prints [:already_tested, :needs_testing]. Which is totally strange. The fail says it knows about already_tested, but then that it doesn't...

Something with modules and inheritance here, I think, since Object has to_yaml defined.

Of relevance:
$ alias spec
alias spec='bundle exec rspec --color'

It turns out bundle exec has something to do with it. Investigating further and will post more here when I find it out.