Disconnect power sources before messing with electrical stuff.
Last night I smushed my extension cord plug and around 2AM I was on the phone with my friend trying to fix it by shoving my soldering iron in there to open on the connections when I felt my body go rigid and I dropped the phone. Luckily the GFI kicked in and kept me from getting my foolish self killed.
Two summers ago, I was in a catering cook tent, and I was running extension cords from a geneartor parked outside. There were two that were unconnected and I guess they'd been lying in a puddle, well when I held the two in front of me, and connected them a huge phosphorescent orb of light was created right before my eyes. I am happy the light did not engulf me or I would be in the matrix now instead of talking about gasoline.