A piece of gravel forced Nico Hülkenberg into a retirement from the Barcelona-Catalunya Grand Prix after it was flicked up by the car in front and hit the safety kill switch on the side his Audi.
Hülkenberg was running in 10th place and challenging for his first points of the season when Liam Lawson's Racing Bulls car ran wide at Turn 12 and kicked up some gravel ahead of him.
The Audi team determined that one of the stones struck the pull loop behind the cockpit that works as a kill switch for the engine, resulting in a complete shutdown of the car.
The switch -- marked on the side of the car with an "E" -- exists on all F1 cars to allow marshals to easily turn off the engine in an emergency situation at the side of the track.
"A very sudden death," Hülkenberg said. "Liam ahead of me just put a wheel into the gravel on the exit of Turn 12 and kicked up a lot of gravel that hit our car and, in fact, pulled the safety trigger on the side of the car that completely kills the entire car. Complete switch off and game over."
Hülkenberg said his car had the performance to challenge for points in Spain.
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"Unfortunately we take nothing on a weekend when we really should have done, like last weekend already," Hülkenberg said. "But this is really unlucky and not much else. I don't know, it's not meant to be somehow and we just keep at it."
Audi racing director Allan McNish added: "It was a very frustrating end to what had been a very good weekend for us as a team.
"Nico's car shut down after Lawson ran wide and kicked up gravel, which struck the safety fire switch. That triggered an automatic safety function designed to shut the car down in an emergency situation. Nico was able to coast back to the pits, but unfortunately that was the end of his race."