That's how i started playing bass. I was the "roadie" (only one they knew with a van who didn't drink) for one of my local bands. Then became their sound man, before borrowing a bass as I wanted to learn an instrument. Couple of years later, the bassist didn't turn up for a gig after going on one of his manic depressive smack binges, so I had to jump in a do the gig. Luckilly the set was your basic beatles style covers so was pretty easy to sit down with the guitarist and a bit of paper writing down the chords, and as i'd been doing the sound for a couple of years I knew the songs by ear inside out. They dumped the bass player, and I took over.
The latest band, 3 of us live within 300 yards of each other so it's pretty easy and doesn't really put me out. Only problem is getting all the gear into a scenic which is why i dont have to do it that often!