Not at a gig but at a practice only a couple of weeks ago...I hadn't touched my gear for months but that's not really an excuse.
I set it all up, it was all plugged in and the effects loop was all set up but I couldn't figure out why I couldn't get any sound. I checked the signal path, the volume was turned up, checked the battery in my bass was ok...still nothing.
Then it dawned on me looking at the back of my rig that there was no speaker cable from the amp to the cab. D'OH!
Also, on my practice combo at home I'm forever putting my bass jack in the headphone socket and scratching my head as to why I'm not getting any output.