Mine both concern weddings.
When I was about 21, I had a couple of friends who had met through our Star Trek fan club. Anyway, when they decided to get married, they asked me (in a casual conversation) if I would come to their wedding. I said I would. I knew their wedding would be held in FemaleFriend's home town, but to me this was an excuse to go and stay with another friend who lived in a city 90 k's away, who would also be coming to the wedding.
So... time goes by, the wedding date approaches and I don't hear anything. Finally I called up FemaleFriend and asked point blank, "Hey, are you going to invite me to your wedding?" She sputtered and I said, "It doesn't matter if you're not, I just want to know so I can get a plane ticket!" So she said she would speak to her parents, who were paying, and call me back.
She called me a few days later and said yes, I was invited. So I went ahead and bought a plane ticket, arrange with OtherFriend to stay at her place. Except OtherFriend did not have a car and it was an hour's drive to FF's home town. However, a guy from OF's work agreed to drive us. OF phoned FF and asked if this was okay, and by the way, could WorkGuy come to the wedding as well because it was a small town and he'd have nothing to do. FF agreed.
So... to recap, FF ended up with two extra guests (me and WorkGuy) at something like 90 bucks a head. And I didn't even buy her a present!!
When my cousin got married she was 19 and I was 20. I honestly thought we were close and kept saying, "You're making me a bridesmaid, right?" She'd laugh and say, "No, no, a flower girl!!" In the end she didn't ask me to be a bridesmaid - she asked the wife of her husband's best man, whom she didn't even know! (I'm still bitter about that but hey, it was her wedding.) That's not the bad part. All my other cousins who were invited had SOs, who were naturally invited. At the time I didn't have one, but I figured I would get an "and guest" and had already asked a male friend of mine if he'd go with me. So when the time came I was invited alone.
Now, having read posts about this, I realise it is perfectly fine and acceptable to invite single people without SOs by themselves. However, being the geeky one in the family (having constantly been asked by toxic relatives when I was getting a boyfriend, why I didn't date, etc - and I did, but I didn't broadcast it around), I took this as an insult. "They obviously think I can't find a man to go with me! I'll show them!" So I phoned up my aunt and told her that I had a date and he'd better be getting an invitation too, because I wasn't coming alone!!
Luckily, my aunt had the stones to say, "Um, that's not how it works. We only have a certain number of seats, and an extra guest would add 150 bucks to the price. So, unfortunately, you won't be able to bring a date."
For years afterwards I still thought she had said that just to be mean to me!! I really don't have fond memories of my cousin's wedding as my brother and I were seated far away from our family (my parents and their brothes and sisters) near the door, which I also took as an insult. I then sprained my ankle on the dancefloor - and no, I hadn't been drinking, I was wearing spike-heeled shoes and there was a sloping edge between the dance area and the carpet and I fell.
I've never felt the same way about my cousin since, and it's sad because she didn't do anything wrong, it was all me!!