The first time I saw the Sweeney revival, Manoel started singing the intro to "God That's Good" (you can't imagine the rapture in store) during "Pirelli's Miracle Elixir." I think he really got distracted because that was the point a TON of latecomers were let inside. And they were not quiet about it, either.
But what was funny was the next time I saw it, Michael Cerveris made the same type of mistake. During the closing, he went back to "What happened then? Well, that's the play..." Poor Patti didn't get to do her last line because of that.
But really, many of these are the types of things only 10 percent of the audience would notice, tops.