It starts out innocently enough: You tell a friend you’ll help her organize her beloved summer festival, or a classified ad seeking musicians sounds too good to pass up, or a fellow singer approaches you at church asking if you’ll perform the Verdi Requiem with her group of 20 singers. But then before you know it, you’re embroiled in a mess of inefficient and wildly disorganized rehearsals, with no end in sight. And you realize, all too late, that you said yes when you should have said no.