I saw KD Lang a few years ago - she stopped half way through a song and asked the person in the front row to stop singing and shouting “I love you” - it took several attempts to shut her up.
We once saw Tracy Chapman in an amazing concert, and there was some idiot near us shouting "Love you, Traceeeee!" throughout - apparently not enough to allow her to do her job properly.
He also kept shouting out the titles of a couple of her biggest hits, presumably thinking that Tracy would stop what she was singing, suddenly change the set that the whole band had planned and practised and smack her forehead at remembering that, oh yes, she had had those huge hits, hadn't she?! - and maybe the audience might like to hear her sing them..... exactly as she was (obviously) going to do at the point she'd decided they would fit into the concert best.