If we're already at the "Oh shit" stage, then we might be happy sacrificing test coverage to go back to known-good code, in which case we would run our fabric `deploy` task and have our rollback completed in ~30 seconds. The idea behind this process is that we never have "oh shit" moments like that :)
(Please see my other response to a similar question, as well)
(Please see my other response to a similar question, as well)