When disruptions are costly, engineers use redundancies to enhance resiliency. In financial markets where many exchanges compete for order flow in the same security, such redundancies may emerge as a positive side effect. We test this conjecture in a large sample of primary exchange outages in European equity markets. Although trading remains technically possible on other venues, the overall market in treated stocks turns illiquid during outages. The effective spread increases by 110% and turnover drops by 96%. We find that the degree of ex-ante fragmentation does not mitigate the illiquidity during outages. Overall, our findings imply a fragility against exchange outages.