I was driving along minding my own business one morning, doing about 60mph in the wet on an open road, when all of a sudden the car began to rapidly slow, as if I had dropped down a gear or put my foot on the clutch. The engine management light came on, but within two seconds everything was back to normal and the engine management light went out a few seconds after that.
At the garage, after the diagnostic computer showed no fault, it was revealed that sometimes going through a puddle can be enough to cool the Lambda sensor (which is attached to the exhaust system) quickly enough for it to throw an error to the ECU. I’m still very dubious as to what this error is supposed to indicate – what possible cause could there be for a sudden drop in temperature other than the splash from a puddle?
So there’s no fault, no repair necessary, and apparently no way of preventing it from happening again. At least it didn’t cost me anything.