These code cause SEGV in ruby 2.1.0-dev but the bug is already fixed in ruby-head (I tested it in ff377d9e81e9c0e58ba1034b1b58cfb68d63c301).

a.rb

#!/usr/bin/env ruby
def let(name, &block)
  ::BasicObject.send(:define_method, name, &block)
  define_method(name) do
    super(&nil)
  end
end

let(:foo) { foo }
foo

with_rspec.rb

require 'rspec'

describe Object do
  let(:foo) { foo }

  it 'causes segv' do
    foo
  end
end