In the '80s I worked in the workshop for a friend who ran a large Garden Machinery company. Every so often, somone would walk down the alley into the yard with either a couple of carrier bags or cardboard boxes. It usually turned out that they contained a dissembled mower or chainsaw that somehow would not go back together.
The first phrase was usually 'I am a mechanic but' or in the case of the ladies ' My husband is a mechanic but'.
This was such a common occurance that there would be a cry of 'CBC' Carrier Bag Customer and all the fitters would run out to the back yard to avoid the FU in the bag (or box).
They would be the same customers who would at length, explain to you what you had done wrong in a winter service that meant they were unable to start their machine in the Spring, it couldn't possibly be stale petrol, OH NO!.