Once purchased a Daihatsu Terios with FDSH and soon discovered we couldn't get the engine to rev above 4000rpm (and it has one of those buzzy high revving petrol engines in). Upon checking the air filter we discovered it had never been changed and apart from being full of fine dirt, it also had all manner of leaves and insects in it as well. A new filter transformed the engine.

On my last FL I don't believe the pollen filter was changed on one of the services despite being an charged/itemised item. When challenged the guy said it had been. Next time the car went in I discreetly marked the items with a Sharpie and checked them later. 2016 BMW X3 30d SE in red
MY2013 SD4 XS Firenze Red, 19" 10 spoke contrast paint wheels, Signature Strips, Grand Black Lacquer trim, Cold Climate Pack - gone!
2007 TD4 S Stornoway Grey - gone!