For years I always took a backup bass and never had a problem that needed one. Never broken a string or anything. Then about 4 years ago my band was playing a gig in Waikiki in a place where the parking garage was only 6'2" tall. My Xterra is 6'2". So I took my small Nissan Versa. Room for one speaker, amp head, gig bag, two wheel dolly and one bass. Shoulda strapped a beater bass to the roof rack.....
I took my Lakland JO4 with Audere preamp. I plugge it in, and distortion. Not the good kind of distortion. I can't get a clean sound. Battery is dying. Look through my gig bag and cant find the spare that I always keep there. Must have used it and forgotten to replace it.
No problem, I'm in Waikiki and there is an ABC convenience store on every block here (sometimes more than one). I go to 4 or 5 of these stores, as well as a couple of other places and all I find is AAA and other small camera type batteries. Not a 9V in sight. Damn. Go back to the gig and the guitar player gives me an old battery he took out of one of his effects. It was distorting a little, but better than the one I had. I play the gig distorted and farting every time I dig in. But I make it through.
So, the first time I don't take two basses is the first time I've ever needed two basses. Just my luck.
Things I learned. Always take two basses (I like to make one of them passive, less to go wrong), or at least everything you need to fix yours, including batteries, strings (old ones will do in a pinch), tools, etc.
I made it through the gig. Didn't sound that good, but got the job done. And had a great time, which brings up something else I've learned. When the "stuff" hits the fan make the best of the situation and don't let it get you down. Just have fun anyway.