This sort of thing happens in the best of families.
A HP Noise & Distortion Test set at one of my workplaces had a rear panel mounted mains fuseholder with its (uninsulated) end within
a few millimetres of an internal shield.
This plate was not mounted very securely,& could flex,with the definite possibility of shorting the mains active to earth/chassis.
Fix?
Glued a piece of fairly heavy perspex on the shield!
Then there was the Marconi Sideband Analyser,where a loose spare nut lodged under a tagstrip,& would on odd occasions short out the 300v Dc HT line.
Or,again Margoni ,Chroma/Luma Gain & delay test set,where they squashed the wiring form between the front switch bank & the PCB under the switch.
Or,the Sony BVM1410 Picture monitor with the 240V & 120V Degaussing circuit PTC thermistors transposed.
The 120V one did last 10+ years,but when it did go bang it went up like fireworks night.
Easy fix,as there was the correct spare----------sitting in the wrong position on the board!