The use of the “conditional” event attribution approach for investigation of an unusual event is worth paying attention to. Most of our built infrastructure is designed to flood frequencies determined from the period of record. It is increasingly the case that such designs will fail. Building a more correct understanding of the reasons for failure will lead to more effective responses. Not all unusual extremes are due to climate change. Some may be due to land use change, some may be mere statistical outliers, and some may be due to an inadequate period of observation history. Correct attribution is critical to correct response.