This is a totally subject style thing (and this isn't a merge blocker or anything), but I'm curious why you chose this formatting over:

``` ruby
expect {
  double.__mock_expectation(:some_method) {}
}.to raise_error(MockExpectationError, /some_method.*stubbed/)
```

I find the indented `to` at the start of the line to look very odd, and a bit divorced from the `expect().to` expression, vs the multiline/nested approach retains the unity of the expression better (to me anyway).