As a former RAF radar technician the number of unexplained it got better on it's own situations was considerable. I think possibly one of the best lines I ever wrote on a job card was this: "-50V line taken to earth via load, fault cleared." the radar used -50V post office relays for all of it's control logic. Since this had to be able to energise quite a number of relay coils it was also capable of sourcing quite high current. The load that was used to short it to earth was ME. Ours was the second shift with two techs on each shift working on it. Total time to fix the fault was 18 hrs, so a total of 36 manhours. No we never found out what was wrong. Getting bit by the electricity is quite easy at 03:00. As I recall it was quite painful that time, a good combination of voltage and available current.
Working on late 50's and early 60's based tech, in the 80's you kind of got used to being bitten by electricity on a semi regular basis. At least nobody ever was got by the 180000 Volt, 120 Amp pulse that drove the radar transmitter output valve.
So nothing surprises me anymore when it comes to faults coming and going randomly.