
Obligatory not my bridesmaid, but one of my best friends got married a couple of years ago, and one of our other best friends was MOH (we were a group of 6 but the bride and MOH were inseparable ever since they both found out they were pregnant within a month of each other). The entire wedding and reception MOH was in the worst mood ever, to the point that her own father told her to stop being such a spoiled brat on what was meant to be the happiest day of her best friend's life. There's only a handful of photos where she's actually smiling, and at one point in the night her mum's dress had broken and MOH actually screamed and shouted at her for it (like it was her fault), in front of so many people.
The reason she was so unbearable? She wanted her new boyfriend to come to the wedding. They'd been together a couple of weeks but he had been shunned by our entire social circle for stealing thousands of pounds from his friends and employer, and the person he had tried to pin the theft on was a wedding guest.