Keyboard failures during performances, that were not the keyboards fault, but power source problems.... Only one for me...and I gave up on it and it's family of keyboards...

I had a cheap ($500) PSR (Yamaha)... it only had 32 polyphony... and when it exceeded the limit (often)... it would make no sound for at least 10 seconds.... It was a nightmare doing one job..

There is no need to carry backups today... Everybody here has a phone and in a pinch anyone of us can finish a night as a DJ..

It has never happened to me... yet...
I can't say the same for my vocal partners... several times over the years... they have a failure (lose voice)..