I can't help thinking BBC Radio Four's All In The Mind could devote an entire series to analysing and explaining this sort of thing!
Especially that trait by which experience and skill is directly proportional to the complexity of the suggested cause ans cure of the breakdown, and inversely proportional to that of the fault itself….
Like:
"Oh no, our prototype test-piece has failed! "
Actual problem: A band-pass filter in the electronic test-equipment chain, set to stop everything above 10kHz (say), and everything below 10kHz.
The give-away? Merely the pattern of the two pointer-knobs on the filter's front panel.
.
"We've measured everything and still can't see what's wrong! "
A 7.25"g loco chassis, a club project, barely struggled to rotate on its air-tests. Result: months of the bench accumulating bits of paper covered with sums and scribbled Reuleaux Diagrams, by first-rate but now despairing craftsmen…
Actual problem: not spotted on erection – no hole in the middle of the exhaust-outlet gasket.
.
One that caught me out:
"We need make a diverter-board to keep the water-depth on the sill, constant.."
Water-mill affected by surges in the stream, shock-loading the machinery and breaking the hardwood "cogs" (millwright-ese for gear teeth) at about £11 each. We identified correctly, needing limit the very thin sheet of water, a fraction of an inch deep, flowing over the sill onto the 22ft diameter X 9ft wheel. The excess would go harmlessly over the weir. Discussions: lots of bits of paper with sketches of gears, screws, hand-wheels…. Luckily it didn't go that far.
Actual solution: A plank supported by simple brackets, on two wooden spacer blocks.
…..
Are you there, Dr. Hammond?