the audience, a true professional. !!

I was playing Toinette in The Imaginary Invalid once. Walking downstage, crossing stage right to left, and the bloomers under my ankle length skirt slowly slid down my legs. The safety pin I had used to fasten them had come undone. I stepped out of them--downstage center-- continued my cross to left stage where I was to sit on the stage next to the wheelchair. I folded up my bloomers and stashed them behind the back of The Invalid, as though they were a pillow. At the end of the scene, he took the bloomers out and tossed them at me.

Nobody in the audience knew that was all improvisation to a costume failure.

but I have never forgotten that one. One of the other actors was so mad at me, because he was in the middle of a bit of business and I totally upstaged him, even though he was the one upstage at the time.

The actor playing Scrooge, wearing a typically long early Victorian-era top hat, had the top off fall off at the end of the tirade at the charity collectors.

Without missing a beat, she picked it up and gave the collectors a very snooty little "mmph."

Off script - but perfect. Improvisation is quite a talent.