Every time I've introduced two friends that don't know each other to each other they've both suddenly developed a very close friendship and effectively gone off together and totally excluded me from things.
First was when I was at college in my very late teens; I introduced a girl on my course to my best friend from school, who was doing another course, and the next thing I knew both weren't talking to me any longer, and if they did speak were just bitchy and excluded me. Then they both booked a girlie holiday together, and the friend from my course invited along some other girls from my course, but not me. I was totally out in the cold.
It's happened several other times between then and now, but fairly recently it's happened again in a big way. I had two friends that both live fairly nearby; one is my neighbour and the other lives on our estate too. I'd been good friends with each of them for several years. About a year ago I introduced them to each other. Again all of a sudden they were both barely talking to me, ignoring any contact from me and not wanting to meet up, yet posting on Facebook about their wonderful nights out, spa weekends away and shopping trips together.
I wouldn't mind friends becoming friends with other friends if they didn't exclude me all the time! It's like I don't exist any longer if I introduce any friends, and I'm getting to the stage where I try to keep all friends separate and know them all from different parts of my life, ie one from the gym, one from work, one from the school etc, as I find it so upsetting when things like the above happen to me.