Nope, doesn't happen to me.....well, not recently.
I worked educational opera for a decade, and one show one singer simply didn't arrive at the school. We all drove separately usually, with several people knowing each role. My boss was supposed to be narrator, and by this point I'd been around for maybe a year or two. She jumped in for a modified version of the show as the mezzo, and I (with a script, per usual for the role), jumped in as Narrator. Only a little blocking, and two parts I had to sing...but not operatically. It actually worked fairly well, and it became a side gig of mine - especially since for that one the "stage manager" mainly did load in/out and no cues during the show. I'd also started off as an acting major, so it wasn't totally out of my wheelhouse, though it had been a while.
As for a union show these days? Nuh-uh. Would have to be in my contract, with a rider, and be paid more. And asked. Though there are several contracts, especially for Theatre for Young Artists (ie, touring kid shows), where someone is hired as an Actor/ASM. They do load in and maybe a few other things, but also act in the show. There are also some ASM/Understudy contracts still around I believe, though a lot of union SMs are really trying to get rid of those.