Colour coding
Not quite in the same league as other comments, but an 80 mile round trip to a "faulty pc" which was fixed by unplugging the cable to the speakers and reinserting them into the correctly matching coloured socket still sticks in my mind..
I hadn't taken the original call.. I think it was the MD, who was not known for his technical prowess. Had it been me, I might have actually done some basic fault finding before sending out an engineer.