TL;DR : my ex had a bad feeling about someone we barely knew. Fixated on him so much that it (among other things) destroyed our relationship. Am now married to the person he fixated on.
I'm really late to the party and this one is a little different, but I've wondered about it for years.
Years ago I was dating a guy who I didn't end up having a great relationship with. He was very popular in our social circle (a DJ at a local club, everyone thought he was a "great guy"). He ended up being very controlling and manipulative. There was only one right way to do many things, we could only hang out with certain people, do certain type of things, and if I was with my family he'd call me a lot asking when I'd be home. We argued most days but it was always hidden and most people thought we were a perfect couple.
I had a few friends mention afterwards that they had an "off" feeling about him, but that's not the interesting thing. Like I said, he was really obsessed with the idea that certain people were a "good" sort and on the flip side of that he would get extremely hostile and weird about others. Sometimes he would do this about people we barely knew or spent any time around, but he would just say he "had a feeling about them."
One of these people was a guy who was a friend of friends, and my ex HATED him basically on sight. I respected his intuition at first, but it became clear he had sort of fixated on the guy for basically being socially awkward and it seemed a little mean that he would be so openly rude to this person for no reason. I didn't have any feelings about this guy either way, and felt like I constantly had to apologise for how my Ex was treating him. When I told Ex he was being ridiculous, Ex forbid me to talk to Guy out of respect for his feelings, unfounded or not.
At this point it was just getting stupid. I wasn't trying to poke the bear but I also couldn't be forbidden from talking to someone on zero evidence. What followed was increasingly immature/erratic behavior from Ex.
For example, when we were out at a club Ex saw me talking in a group of 4 or 5 people of which Guy was one. Ex left me at the club and Guy, noticing an acquaintance in need, offers a ride home. Ex and I are invited to any party where Guy is, Ex refused to go. Other people began to notice how hostile Ex was and tried to talk him around to no success. Ex didn't like Guy and had no problem showing sides of himself he hid from our other friends, like yelling at me or ditching me places with no ride.
It was so incredibly petty, and caused Guy to notice what a douche Ex was and start to worry about me. This escalated until i no longer cared that Ex didn't like him, and Ex started following me and telling me who I could and couldn't see even beyond Guy. Ended up being a pretty dramatic break up to what was already a really dysfunctional relationship.
In the end Guy and I wouldn't have had any reason to have more than small talk if Ex hadn't constantly fixated on him, but the situation brought us together and we started dating a few months later. Almost 10 years later and I'm married to Guy and we have a kid.
So a big shout out to my Ex. I have never known anyone I get along with and click with so well as my husband. We have so many weird things in common and even our families are very similar and friendly. I often wonder what Ex saw that I didn't all those years ago, because Husband is very quiet and none of these things were obvious then. Husband is everything Ex wasn't and so much more, it's crazy to me that Ex somehow saw the threat in him before I was even remotely interested.