The one where the groom was a close friend and was marrying someone none of us could stand. (I should perhaps add that we were all teenagers, the couple were 18 & 19, so lacking in maturity).
We spent 2 hours in the pub beforehand getting him pissed and trying to persuade him not to go through with it. The bride was waiting in the church porch when he finally rocked up, approx. 20 minutes late. The church was packed and there was some disruption as we (approx 10 of us) struggled to find somewhere to sit.
The reception was at the bride's (huge, posh) house. Almost everyone on the groom's side was pissed or well on the way. There was heckling during the speeches. The best man had a row with the bride's mother, who slapped him round the face. He called her a cunt and her brother laid into him and a brawl developed.
By this time, the bride had locked herself in her bedroom, hysterical and threatening to slash her wrists. I don't remember what happened next, because a large loudspeaker fell off its bracket on to my head, and I was in hospital having 12 stitches put in. Apparently, I had managed to bleed all over the living room, hall and stairs, and the cream leather upholstery of the Jag some kind person took me to hospital in.
My BFs lovely dad drove miles to pick BF up from the house, then to the hospital to get me, and take me home. I had concussion and felt dreadful for days.
The marriage lasted about 13 months. She had an affair and he left her.