In Godspell, which we just did in my theatre, the first number involved a set of wireless mics, one with an echo effect and the other without it. We knew the mics were unstable, and we had to guard it with our hand on the master slider if something happened... anyway... during the second show, (second show is always worst), everything seemed to be going well..it was a quiet part of the number and then.... KA-FOOOOOOOOOOOM!!!!!!!! feedback. I can't explain to you how loud and startling this was, we gave up on the mic for the rest of the number.