1) More recently than I'd care to admit, I had just finished setting up for a gig. For reasons I don't entirely remember, I decided to switch cabs (top and bottom). I unplugged the cabs, moved the head, re-stacked the cabs, and (thought I) plugged the cabs in again (two cords from the head). Turned on the head, walked away with my wireless, and couldn't figure out why I wasn't getting any sound. Tried playing some more across the room when I got a couple of nasty pops. The singer said he saw sparks, and I thought he was kidding. Nope -- one of the cables had contacted the G string on my second base, which was on its stand next to the amp. Burned completely through the string, and scared the crap out of me. Fortunately both amp and bass were fine; string did not survive.
2) Testing out my new Sennheiser wireless at home a few years ago, I made the mistake of turning off the transmitter first. Now there's a piece of yellow tape across it saying "FIRST ON, LAST OFF!"
Trust me when I say neither of those is a mistake you make twice.
-jb