A very good question, hence my original post, now im hoping youll answer that for me!
Whilst im aware you could DIE if you're changing say the upstairs light fitting when your trusty wife comes running up the stairs and turns the landing light on, obviously not aware the borrowed neutral in your hand runs from the downstairs lighting circuit through the landing bulb and now through YOU.
MY practice is i normally cut the MCB for the circuit im working on and then test again.
Again this seems a very poor practice.
Im having a new CU fitted next week by a qualified Sparky, so from then onwards i think ill always trip the RCD for that rail first just in case.
Talking of bad practice:
There's many more ive seen below. I wasn't even aware of this one and i need to read up on it:
"Running cables through insulation such as loft insulation with consideration of cable de-rating factors."
Adding more than one 13A twin socket as a spur off an existing ring socket.
Wiring sockets with Line and Neutral reversed - incorrect polarity
Omitting to use rubber grommets in metal back boxes.
Failing to mark up cpc (circuit protective conductor) with green/yellow sleeving when extending circuits.
Using incorrect cable size.
Installing light fittings in bathrooms and shower rooms which are not suitable for the zone.
Installing light fittings and light switches which require an earth into a lighting circuit without a cpc (circuit protective conductor).
Running a cable outside of permitted zone e.g. behind skirting boards or diagonally across walls.
Running cables through insulation such as loft insulation with consideration of cable de-rating factors.
Installing ventilator fans without manufacturers recommended fusing. E.G. Kitchen fan spurred off a 10A lighting circuit but fan requires a 3A fused supply.
Wiring a cooker hood without the correct in line fusing same as for fans.
Using a junction box with the wrong rating, e.g. teeing of a ring 32A ring circuit using a 20A junction box.
Using a junction box under floor boards (screw terminal connections should be easily accessible for inspection and testing).
Inappropriate use of flat twin and earth cable in outdoor applications. Cable is not approved by BASEC - British Approvals Service for Cables for external use.
Installing circuits into garden (pond pumps/lighting) without any RCD protection.
Not proving earth continuity back to main earth terminal.
Trapping wires when screwing covers back on (yes, electricians can do this too but an insulation test should indicate a problem)
