I was in a play in high school where, on opening night, we accidentally skipped most of an act. There was a key line that ended the act, and the actor delivered it in one of the first scenes. Fortunately, the guy playing the lead was an absolute master at improvisation.
The funniest, though, was a performance of Our Miss Brooks. For those who don’t know, the play is set in a school, and the students are putting on a play-within-the-play. One girl panicked because she had forgotten her line… The line in question being “I’ve forgotten my line!”.