Every time I introduce a friend to another friend they become best buds very quickly and I am excluded. It happened to me at college years ago, and has happened twice in adulthood. Most recent being in the past 6 months.
The two I recently introduced to each other were my closest friends. Now they are off doing stuff together all the time, they don't invite me and barely talk to me anymore. Whenever I suggest anything to either of them they are always busy and make excuses. I was a good friend to them both.