In 1939, Swiss physicist Walter Jaeger was trying to invent a sensor for detecting poison gas. His device was supposed to react to gas by detecting a drop in ionization, thereby triggering an alarm. 

However, when Jaeger tested the device, it failed to detect the gas. Frustrated, he lit a cigarette and noticed that the smoke from the cigarette caused a reaction in the sensor, reducing the current in the circuit. 

This unexpected result led to the realization that the device could be used to detect smoke, not just poisonous gases. This serendipitous discovery eventually led to the development of the modern smoke detector.