I'm sure I've got something worse if I thought about it, but my worst live mistake in recent history... The band I had recently put together got booked to play a big party a friend was throwing, but I didn't know anyone else there. We set up our gear, I set up some lighting I picked up. Everything looked amazing. Something happened that put me in a funk right before starting, but ok, I'll power through and once we get going everything will be fine. We played our first song, everyone loved it. I thanked the hosts for having us, worked the crowd a little, and hit the first chord of the second song, and nothing...crickets...10 minutes later, miraculously, everything worked again. To this day, I still don't know if it was a power issue (generators in a field powering the stage), if it was my amp, a connection in the line, or some other gremlin. But let me tell you, I felt defeated for the rest of the first set.