My (now ex-) Mother In Law decided back when we were in High School that I wasn't good enough for her daughter. It got problematic enough that we stopped dating and didn't start again till we were both away at college.
During college (after several years of dating not under mom's eyes) we decided we wanted to marry each other, but couldn't reconcile the fact that she absolutely wanted children and I absolutely didn't. It broke our hearts, but we decided it was irreconcilable, so we broke up.
Fast forward a decade, she's got 2 kids (one barely a newborn), and I learn she's divorcing their dad because he was having an affair and it started before, and continued well after she became pregnant. So, I ring her up and tell her I've had a change of heart regarding kids (I still don't "want" them, but it's no longer a hard no for me) and we start dating again.
10 months later we marry... and it sets in with the MIL. She started in with the typical "nice to your face, but shit talk anything they can think of about you behind your back".
After a year or two of it my wife had had enough and she basically wanted to tell my MIL that she married me because she loved me, I was a good husband, a good stepfather, and a good provider (I earned more than we needed and she could stay home, while still saving for retirement and living a comfortable middle-class life); and that if that shit-talking me all the time didn't stop then she (MIL) was going to lose her daughter and grandkids. I told wife not to do it because it'd hurt the kids losing their grandmother... we could just ignore her being a bitch.
Turns out that after enough years of my then-wife having a slow but steady constant reminder of how I might not be ideal she started to internalize a little bit of it.
Then her mom fucking introduced her to someone (not so obviously, just like "we hired an off duty police officer to work as security at our church and it turns out that he went to school with you, is a deacon at his church, and runs his kid's boyscout troop, do you know him?"). And that was the beginning of the end.
I later learned that her mom was covering for her cheating on me with aforementioned dude and was coaching her for how to get as much out of me as she could before dropping a surprise divorce on me.
TLDR: Ex-MIL shat on me to my ex-wife behind my back for years. Ex-wife wanted to cut relations with her, or put her in her place, but I advised against it. It was bad advice, I lost my whole family after my ex-MIL strategically inserted a man she approved of, then helped cover the "transition".