For me it was probably my sister's wedding. Being her only sister I was, naturally, chief bridesmaid. However, she'd either omitted to tell her best friend, or the info was ignored. Just because BF was 10 years older than me and the other two BMs was neither here nor there.
Luckily I knew trouble was brewing. We got to the church and BF tried to take over things pre-ceremony. No one had told us what formation we were walking in, so I decided to use one that my sister had mentioned and have me at the front, the other two together and BF at the back, to keep her out of the way! However, at the end of the ceremony as she was at the end of the pew nearest the aisle she thought she should have the bouquet to hand back to my sister, and then walk down the aisle with the best man. Cheeky mare! I refused to let go of the bouquet and instead told her to move back down the aisle to let us line up as we had before. She was not amused!
She didn't give up, mind you, and told the best man at the reception that they'd be taking the first dance together. He told her that no, he would be dancing with me! Honestly! got to admore her determination though!
In the loos later on I overheard the gooms granny slag my mother off because the wedding cake wasn't going to be cut until the evening, when the evening guests had arrived. Fair enough, she'd made the cake, but it had nowt to do with my mother, it the was B+G's decision! I did get a rather embarrassed apology from the groom's mother when I emerged from the cubicle, though!
Other than that, there was the time I was BM for a friend of my sister, and my sister, not liking the fact that she wasn't the centre of attention, stormed out the reception moaning that her boss had been molesting her. No idea if it was true, but surely not the time to bring it up. The only memory I have of that day is of my folks trying to calm her down in the carpark and me getting sent back inside. :-(