This was a (relevant) part of an article which appeared on Playbill’s website.
How Eva Noblezada saved the day by letting Tam performer pee on stage
Eva was only 17 when she starred as Kim on the West End, and holy cow, she was so good.
James and I saw her on the West End and flipped out. Of course, I love a good onstage mishap and she entertained everyone in Scottsdale with several Miss Saigon doozies.
During the final scene in the show (spoiler alert), she is with her son, Tam, before she dies. The boys who play Tam are really young, and she noticed that this particular one was squirming up a storm, aka doing “the pee pee dance.” He had to go!
Well, she kept him focused during their exchange, but as soon as the light left them and went to the other side of the stage, she told him to let loose. Yes, she told him to literally pee in his pants… and since they were in an embrace, that also meant peeing on her. When she died, she smartly faced away from the audience so they wouldn’t see all the wetness, but she reenacted John’s reaction to discovering her body.
If I had to analyze it, I’d say it went from devastated (as the script calls for) to disgusted (as anyone would react to seeing an actress soaked in pee). She admits the smell wasn’t pleasant, and apparently, John confirmed that. I guess the ‘M’ in Miss Saigon should have been changed to a ‘P’ that night. You do the math.
This part, and full article (c) Playbill.