Bands reacting to disasters on stage.
 

It is a known fact that disasters do happen, no matter what counter measures you've taken. It's a testament to a band's professionalism how they deal with equipment failure a band member collapsing or other disasters and not freak out.

So let's talk about that.

First of all, people always look down on when a guitarist takes along multiple instruments "He can only play one at a time!" is the argument most often used. But this clip of legendary guitarist Stevie Ray Vaughan shows, there's a very good reason on why a guitarist at the very least has two instruments on stage.

A string breaks, he plays on (LIKE A CHAMP!), finishes his solo while nodding to his crew to hand him another guitar so he can finish the song.

But having a capable road crew is no guarantee that a guitar switch will work.
This is the Dutch Ska band Doe Maar and pretty much from the get-go the guitarist has issues with his guitar, during the first chorus he nods to his crew that he needs his back up, during the second verse, there's no guitar because they're fumbling around trying to switch instruments. When the thing is finally reconnected, the Bass player goes "Hey guitar!" but the poor guy isn't out of the woods just yet because when he tries to play the solo, a string snaps making the guitar go completely out of tune. The sax player quickly steps in and does the solo for him.

On Iron Maiden's 'Somewhere Back in Time' tour back in 2006 or so, the Power to the stage completely shut off for a while, and instead of just standing there or going back stage, they take a football from back stage and decide to have a little kick around onstage until the power went back on:

I saw a 90s band called Lit at a festival in Naperville, IL. The power went off for a long enough period that the crowd got uppity, so they grabbed some acoustic guitars and huddled as many people together at the front of the stage to play a song until the power was brought back on. It was a noble as hell effort.

That SRV video at the top is legendary.