I went to a newish block of 6 flats to test the communal emergency lighting, found the test switch and turned it off, after the test none of the lights worked again. Turned out that the test switch cut the permanent L but not the switched L, the electronic gear didn't like that at all.
I tried cutting all the power for a few minutes, new lamps, and even swapped batteries in one but it was no good. All the lights lasted the full duration though... Had to go back and connect the test switch properly and replace all the lights. While I was there I even went as far as fitting earth sleeving to the bare CPCs and blanks in the DB. This building also had some fake wiring where a switch and outside light were installed, but no cable between them. The things some people get away with!