Friend of mine was in the MIDDLE of her set at a festival last year, when one of the audience (who is a no more than, and possibly less than average floor singer) came up and gave her suggestions on what she considered was going wrong ! Had the same sort of thing happen to me in the middle of a show I was doing last year - in this case a friend came up during the interval and told me that I was talking too fast in the links. (Little did she know that I was having to do that as the first half was running far too long in rehearsal). In her case I think it was a general attempt to be helpful BUT not great for boosting one's confidence, and more than a little rude in my friend's case, I would have said!! There's something to be said for training in Audience Technique I feel.............