Totally true story, my eldest Son Jamie (now 45) was on work experience at our then local Vauxhall main dealer, Davenport Vernon, long gone, now a Frank Sytner BMW Garage. Jamie as a kid had a mechanical bent, so much so that whilst on work experience he was entrusted with fitting replacement disc pads whilst under some sort of supervision, but obviously not enough!
The pads must have been symmetrical because he managed to fit them back to front. It was only when the customer brought the vehicle back very soon after collecting it, complaining of noisy ineffectual brakes that the error was picked up. D.V. covered it up with excuses about faulty pads etc. corrected everything and the happy customer was on his way, and my Son was very embarrassed. Absolutely shocking really, thank goodness there were no more serious consequences.