The situations can differ. Sometimes the band has been established already, and maybe they just need a sub for one show, or maybe someone quit or got fired, but gigs were already on the calendar. It can be any number of things. I have done many many shows with little to no rehearsal. A few of them I had only met the band when I showed up for soundcheck. They don't always go south, it really depends on the professionalism of the members. If everyone is well rounded, and did their homework there's no issues.