One other behavior that might be worth spec'ing is this:

``` ruby
it "does not affect the global random number generator" do
  srand(123)
  val1, val2 = rand(1000), rand(1000)

  srand(123)
  subject.order(items)
  expect(rand(1000)).to eq(val1)
  subject.order(items)
  expect(rand(1000)).to eq(val2)
end
```