This is partly being written as a warning and also to ask if anyone else has experienced it and should it be expected to be found by any normal inspection.
We have just, last few days had a kitchen area replastered, therefore there was a lot of condensation in the room forming on various surfaces. One of these surfaces was the light switch, plastic front not metal. When I went to operate the switch it was very stiff as the plasterer had managed, as they do, to get plaster all over it and some was jamming it. So as I applied more than normal force to it my finger slipped off and caught onto one of the fixing screws, ouch, I got a reasonable belt off it, enough that my arm was numb and tingled for a while afterwards. On investigating it the problem was that although the earth wire in the cable was connected to the brass screw terminal in the back box the terminal was fractionally loose where it was crimped onto the box. Over a period of time this crimp appears to have corroded, possibly due to moisture in the kitchen over the last 25 years? Anyway using a DVM, the Meggar having vanished for the day, I measured a 100K, or there abouts, resistance between the screw terminal and the back box, therefore the box was not earthed and that's why it hurt! Moisture having provided a path from the live on the back of the switch to the metal of the box and hence the screws.
I'm not looking to apportion blame any where, apart from maybe at myself for having touched a damp switch, more just interested in if it would be reasonable to expect the fault to have been picked up on a routine electrical inspection or is this taking it too far?