Refitting floorboards properly, prior to laying the bathroom floor.
Marked on the board where the cables underneath where (old installation, a bunch (about 8) of cables running together through a notch in the top of the joist, over about 2/3 width of the board).
Then promptly put a screw through where the cables were (it was late, I was tired)...
surprised there wasn't bangs, complaints of all the power going off etc.
Lifted boards again, thinking I'd then got to faff about repair the cables.
it had indeed gone through two cables, but it turned out that they were both redundant cables as there were disconnected ends floating about- (not far from the CU, they must have been disconnected at some point - think they were probably for old storage heaters