I have never been fired from an SM job, but I have worked with an SM who was replaced. It was a strange situation too.
She had just graduated from college and she interned with an Equity house. The house needed an SM for their winter show, and she was gifted her card. I personally think it's a bad idea to gift a card from someone who has never stage managed outside of college. But the company did it. I was brought on as ASM for the show.
Once we got into previews and then into runs, she had made the mistake of missing sound cues on multiple occasions. The company decided they didn't want her to call the cues anymore. Being an Equity house, they had to have an Equity SM, they didn't want to try and find another Equity SM, so they ended up changing her title to PSM and then bringing on a calling stage manager. Ironically, the calling stage manager was one of the people I had competed with for the ASM position. The whole scenario made the management team a bit uncomfortable. The actors weren't happy with the Equity SM, weren't told she wasn't calling the show anymore until half hour before the new calling SM called her first show, even worse, at the end of the production, our lovely actors brought gifts for the director, playwright, etc. I was the only one from management that was given a gift from them. It was just a bad and uncomfortable situation to be in.
Another situation I was in, I again, wasn't fired, but I wasn't re-hired either, even after a conversation with the PM about it.
I was working as an ASM and I had an ASM intern working with me. She was quite difficult to work with. She was consistently late, to the point that I had to do her pre-set work on top of mine. She would primp backstage during the show instead of taking care of situations that arose during the run. She was horrible at quick changes and made an actor late on stage every time she did the change versus me doing the change.
I reported this to production management, it was getting out of hand and ridiculous when she started showing up 10 minutes before start of show. My production manager took it as me "tattling" on a member of the staff and said I shouldn't have done it. (This is after I has reported it to my SM and my SM did nothing either) I told her politely that being an intern, she needs to be told this stuff and it wasn't my place to do so, and it needed to come from above, but the above wasn't there to witness performance and I couldn't always do her work.
This conversation led to my contract not being renewed the next season. The ASM intern was hired, on though. Ironically, the PM was fired and replaced. The new PM not knowing the history, realized the intern, now SM had problems and fired her and I was contacted to replace. (I didn't, the company was a mess because of the firing) Then the intern moved onto another company as ASM, and then was replaced in tech week for that show also.
So, even though I wasn't renewed, I'm not upset about it!