Since I tend to think that I make too many mistakes playing live, I would never think of calling someone out for a mistake of their own in front of an audience. Having said that, even if someone on the stage is making terrible, terrible mistakes, pointing it out on stage, trading insults, or making comments about it to the audience is never a good move. Bite your tongue, get through the set, and then have a talk. If the problem is so egregious, get through the night and then let the offender go. Life's too short and even if it's a job, music should be fun and enjoyable... right?