Painting house. A few dangling wires under the eaves (and an indoor 3 pin socket I had no idea was there). Electrician had previously assured me that none of the old white dangling wires were live (mostly coax anyway).
Wife is up scaffold getting ready to paint and is removing wire bird mesh from beneath the eaves using electrical snippers (insulated). She spots a dangling wire and asks me if it is OK to cut it. Yes. (Scaffold is on big rubber/plastic wheels and anyway the sparks said the external wires are dead). She cuts and pushes the tail back into loft space. Painting proceeds.
Much later, she tells me the upstairs lights are not working. RCD controlled circuits upstairs and MCB for light circuit upstairs both tripped. I had turned the gates off earlier (RCD protected) and for some reason thought that caused the trip and so reset everything). We go to sleep.
The next day I have a nagging worry and concerned about fire risk go in search of the cut tail in the loft. Eventually find it and pull it up from beneath insulation. The wire is live. It does not feed anything that is in existence now. I normally test all electrics with my megger. However, I relied on someone else. Could easily have had shorting across the exposed ends and set fire to the insulation in the loft.