On opening night of the Kennedy Center Sweeney Todd with Brian Stokes Mitchell, the understudy Mrs. Lovett forgot a line during "A Little Priest" -- after Sweeney says "I prefer general", she completely forgot "With or without his privates?", and of course 90% of the audience knew the show by heart and knew she'd dropped the line. She ad-libbed something, though, the conductor kept the orchestra vamping, and she and Stokes went through to the next chorus -- but THEN they went back to the earlier point, ran the lines again only marginally in character, and this time Mrs. Lovett nailed the "With or without his privates?", at which point the audience exploded into applause and Stokes started laughing so hard that he fell off the stool he was sitting on. One of the funniest moments I've ever seen in performance.
"Sweet summer evenings, hot wine and bread /
Sharing your supper, sharing your bed /
Simple joys have a simple voice:
It says why not go ahead?"