Every engineer knows that anything that can go wrong will go wrong. Breakdowns will come, as Murphy’s Law outlines, at the worst time, in the worst way. That’s why so much effort goes into avoiding so-called single points of failure – system components whose collapse can bring down the entire system — because engineers know that, sooner or later, those critical elements will crash.

London’s Heathrow is providing a real-life lesson of what happens when Murphy’s Law collides with a single point of failure: utter chaos. A fire erupted at a major electrical substation on Thursday night, triggering a loss of power across a large area. By Friday morning, the airport — Europe’s busiest, and the world’s fourth most active travel hub — had shut down completely for the day, the UK’s worst air travel disruption in years.

The airline industry is already fuming: “How is it that critical infrastructure—of national and global importance—is totally dependent on a single power source without an alternative,” asked Willie Walsh, the head of the airlines association and, before that, the boss of national flag carrier British Airways. He has a point — but protecting those single points of failure is easier said than done, and is subject to a cost-and-benefit analysis.