We describe metastable failures-A failure pattern in distributed systems. Currently, metastable failures manifest themselves as black swan events; they are outliers because nothing in the past points to their possibility, have a severe impact, and are much easier to explain in hindsight than to predict. Although instances of metastable failures can look different at the surface, deeper analysis shows that they can be understood within the same framework. We introduce a framework for thinking about metastable failures, apply it to examples observed during years of operating distributed systems at scale, and survey ad-hoc techniques developed post-factum for making systems resilient to known metastable failures. A systematic approach for building systems that are robust against unknown meta-stable failures remains an open problem.