In our 70s vintage house there is a light switch on the wall (not near
the door) with a piece of red tape on it, obviously saying "Do not
switch me". I decided to see why. It turns out to be one of two
switches (the other at the door) controlling the main room light. I
tried switching it when the door switch was off. The light came on
briefly as the switch passed the midway position, then went off again
when the switch was fully switched. In other words, both positions of
the switch left the light off. Hmm. I removed the screws holding the
switch plate, and replaced the switch with an identical one. Same
behaviour. Hmm. I then discovered that I could make the light go on
and off by moving the switch plate slightly (when off the wall). Sounds
like a wiring fault, but unfortunately the wires between the end of the
sheathing and the switch are clearly intact. Hmm. Out with the
voltmeter, to test the voltage between phase and earth and see if it is
affected by movement of the cable. This is where it gets really odd.
The voltage between the hot wire and earth cycles between 230v and 190v
with a period of about 7 seconds. WTF??