If you've read my rant about people talking and singing along at shows, then you know that this is the place to post your horror stories and suggestions for how to stop it. (If you haven't read the story, you can find it here.
I can tell you that it's a pretty widespread problem; we caught a performance of the revival of "A Chorus Line" on Broadway recently, and a woman behind us was singing along with some of the numbers. I think the performers onstage were doing fine without her assistance. Anyway, to post your own stories, use the comment section below.