One benefit of this scenario is that as bassists, we're not usually the focus of the piece. Unless the bass line is an iconic one or is otherwise cast in stone, we do have more freedom than the drummer or rhythm guitarists often do. This is not as excuse to skip your homework and show up unprepared, but let's face it, none of us knows every song ever written. You will at some point encounter a phrase, chorus or change that has you stumped. It's at that point where you realize just where you stand: Champ or Chump. How quickly and smoothly you can become invisible again is the key. If you're onstage with the best soloist who ever lived and the song is cranking along, everybody is loving it and you keep it truckin', chances are after their great solo garnered a standing O, they won't point at you and call you out for playing a fifth instead of an octave on the sixth note of the third bar of the second verse.