Practice makes Perfect
As far as lining up the songs you will perform at a certain gig, it remains you have to practice the piece in order to make sure nothing goes wrong. There is not much room for impromptu rendering as clubs and organizers who get bands would expect nothing but the best from bands today.
Some would take their gigs for granted, believing that they are too good to practice. That attitude will get you into trouble as you would never know when any instrument used would be off key. Bands have all the time in the world to practice. It is just a matter of discipline.