Start ruby debugger if rspec test fails

You won't get access to local variables (easily) without debugger being in scope of the block, however RSpec provides you with around hooks which let's you do this:

config.around(:each) do |example|
  result = example.run
  debugger if result.is_a?(Exception)
  puts "Debugging enabled"
end

You then have access to @ivars and subject / let(:var) contents at this point.

I like @jon-rowe's solution (no additional gems needed) with a slight edit: I really don't care about other errors as much as RSpec::Expectations::ExpectationNotMetError.

config.around(:each) do |example|
  example.run.tap do |result|
    debugger if result.is_a?(RSpec::Expectations::ExpectationNotMetError)
  end
end

Use pry-rescue, it's the spiritual successor to plymouth:

From the Readme:

If you're using RSpec or respec, you can open a pry session on every test failure using rescue rspec or rescue respec:

$ rescue rspec
From: /home/conrad/0/ruby/pry-rescue/examples/example_spec.rb @ line 9 :

     6:
     7: describe "Float" do
     8:   it "should be able to add" do
 =>  9:     (0.1 + 0.2).should == 0.3
    10:   end
    11: end

RSpec::Expectations::ExpectationNotMetError: expected: 0.3
     got: 0.30000000000000004 (using ==)
[1] pry(main)>