Several.
At one I was the flower girl (so still pretty young, around 8) and the wedding was being held at a country club in Los Angeles. It began raining (which NO one was prepared for because LA) and the groom wouldn't allow the ceremony to be moved inside. He and the bride got into a full blown fight, like running down the hallway screaming at each other as we all sat in the reception hall eating mints quietly listening. The wedding was called off and I could hear the bride sobbing down the hall as we left.
A couple years later I attended a wedding that was going really well until a guy (I think a cousin?) grabbed the mic and asked for everyone's attention. Instead of giving a nice speech for the bride and groom, he called his girlfriend out to the dance floor and proposed. The rest of the evening was incredibly awkward and all anyone could talk about was how rude that guy was. I was maybe 11 and knew it was a rude thing to do.. Just embarrassing for every person involved.
A couple years back my husband and I attended a wedding where the groom wrote a song for the bride. Sweet, right? Yeah, the whole song was about their relationship and how he didn't call her back after their first date, began dating someone else for two years, and then when that chick left him he got drunk and called the now-bride for some late night lovin'. The weird part was that the bride didn't seem to understand (or care) that she was the after thought back-up pussy - literally years later. We were sitting near the brides mom and she didn't look pleased.
This last fall I attended a wedding where the maid of honor went full white girl wasted and shared WAY too much about her and the bride's past. Sneaking out of the house, drinking underage, and threesomes with other men. I have no idea why the mic wasn't taken from her and why she was allowed to babble on for 15 minutes. The groom's parents were sitting at a table near mine and we could hear the groom's mom saying, "What the fuck" over and over. The MOH ended up causing a scene later that night yelling at the other bridesmaids about "not being good enough friends" for the bride. In truth, the MOH was the only person in the wedding party not married or engaged or in a relationship, so I think she was really having a rough day.
I'm apparently terrible luck at weddings.