I haven't been in any plays or musicals but I was on stage once as sort of "emergency cover". And today is the one year anniversary of that funny accident so I decide to share.
So I was the manager of our school orchestra last year. I was only in charge of general management but at the last rehearsal before our performance the conductor suddenly said that he needed me on stage. He gave me the harp score and asked me to practice it on a piano. I was like "you know this is for a harp
and as someone who has never rehearsed with the orchestra I can't even get the cues and there's only one week to the concert, you do realize that right?" and he was like "yeah but the thing is we don't have a harp. I have to tune a keyboard to fake the part so just do it." OK then. I spent the entire week on the score. Honestly I never understood how conducting works so I just memorized my part by listening to audios. I was so glad that I didn't screw anything up in the concert which was truly a miracle.
There were other funny mishaps too. On the day of our concert the stand of a drum was missing. We searched everywhere but just couldn't find it. So we ended up putting the drum on a chair we stole from a lecture room and it looked really awkward
on stage. Also during one song our conductor was somehow going faster and faster. Some players were following him while others chose to stick to the correct tempo. And the song turned out to be a total mess.