that can definitely happen! my band lost its lead guitarist a couple weeks ago and we had to play a paying gig last saturday. we hustled up a new guitarist post haste and came up with a set list combining the songs we already knew plus about 15 songs he knew that we didn't know. we only had time for a couple practices so we went into that show just barely knowing most of the music, but you know what? the show was a big success! we got through it by doing a lot of jamming, improvising, and looking at each other for visual cues. there were some flagrant and outright hose-ups even on my part, but believe or not no one in the crowd cared! i'm always amazed at how forgiving a crowd of drunk people can be as long as everyone in the band is grooving and having fun. as was already mentioned, it's important never to stop playing because of a mistake, and to never make a face, glare at your bandmates, or do anything to let on to the crowd that anything's wrong. you can power through an awful lot that way.