Using Rspec's "let" inside an "it" block. Crazy? Yes. Useful? Occasionally.

some_spec.rb

  describe "#mentions_story?" do
    subject { described_class.new(file) }
    let(:file) { "COMMIT_EDITMSG" }
    before do
      File.stub(:read).with(file) { commit_message_text }
    end

    context "commit message contains the special Pivotal Tracker story syntax" do
      it "matches just the number" do
        let(:commit_message_text) { example_commit_message("[#8675309]") }
        subject.should be_mentions_story("8675309")
      end

      it "matches state change and number" do
        let(:commit_message_text) { example_commit_message("[Fixes #8675309]") }
        subject.should be_mentions_story("8675309")
      end
    end

    context "commit message doesn't contain the special Pivotal Tracker story syntax" do
      it "doesn't match without brackets" do
        let(:commit_message_text) { example_commit_message("#8675309") }
        subject.should_not be_mentions_story("8675309")
      end
    end
  end

Why not just create a method to call instead of the lets? I don't think they add any clarity to the examples. Something like this:

describe "#mentions_story?" do
  subject { described_class.new(file) }
  let(:file) { "COMMIT_EDITMSG" }
  def stub_commit_message(message)
    File.stub(:read).with(file) { message }
  end

  context "commit message contains the special Pivotal Tracker story syntax" do
    it "matches just the number" do
      stub_commit_message(example_commit_message("[#8675309]"))
      subject.should be_mentions_story("8675309")
    end

    it "matches state change and number" do
      stub_commit_message(example_commit_message("[Fixes #8675309]"))
      subject.should be_mentions_story("8675309")
    end
  end

  context "commit message doesn't contain the special Pivotal Tracker story syntax" do
    it "doesn't match without brackets" do
      stub_commit_message(example_commit_message("#8675309"))
      subject.should_not be_mentions_story("8675309")
    end
  end
end