One of the funniest things I've had go wrong was years ago. We had no money and I'd rigged up our cheapo, secondhand lights to a homemade footswitch that when pressed it would switch off the normal coloured lights and switch on the smoke machine and strobe behind the drumkit, we used it mainly for our cover of Overkill by Motorhead. It got to that point in the song for strobe and smoke to accompany the drums (if you know the song you'll know where I mean), so I over enthusiastically stomp on the switch and break it with the smoke and strobe going, I couldn't turn it off without unplugging from the wall at the end of the song, by which time the stage and most of the dance floor had completely disappeared from sight. The crowd loved it, nobody had a seizure and it was our last song so it all ended well but it did take quiet a while to clear enough to be able to see anything
You don't always need double of everything, just make sure what you do have is suitable for the task