I was working on a block of units that had recently been renovated and was up on the metal roof with a sheet lifted trying to find a lost cable. At the same time the apprentice is having a bludge leaning against the aluminium ladder I used to get on the roof. I pulled on a cable and found it was cutoff. Thinking it was disused I threw the end of it onto the roof. In fact it was live so now the whole roof is at 240V. I didn't notice because I was standing on top of the roof but it certainly made the apprentice jump.
