When I first started getting into wire work, it was due to rewiring my house. Pretty simple gig, we've all done it - kill the circuit, disconnect any fixtures/switches/receptacles, hook the new to the old, pull on one end and the new wire follows the old through the conduit - reconnect, and PRESTO!
I had killed the circuit to my Dining room, and was replacing the old linen covered, rubber insulated wire for that room with shiny new 12AWG Cu THHN/THWN (Ooooh, new wire smell
Well, it was going well, and getting late in the evening (summertime). Was connecting my last run of the day to the existing conductors for power-up, and connected the new white THHN to the existing "neutral".
Well, what I didn't
know is that the previous person working on the line (owner? electrician?) had "piggybacked" the living room
"neutral" onto the one for the Dining room - and that circuit was still powered, and under load. When my strippers broke through the old rubber insulation, while grounded to the j-box...
<<POP>> a blue flash, and there goes the night vision!
Luckily, I was working above my head, and not looking directly at the arc... but the "backflash" off my surrounding was dazzling. I went to the panel and killed the main for the last 10 minutes worth of work by flashlight.
Of course, my "12 AWG" strippers were now about an "8", but hey, who's counting!