I was a guest at a very good friend's wedding years ago, where the groom's sister made no secret of being upset it wasn't her turn to get married. She'd had two children from two very short term relationships and her mum would tell anyone who listened that it was a disgrace that neither man had stepped up and married her, as well as being the oldest of 4, where this wedding was the last of the younger siblings to get married.
Anyway, shortly before the wedding, she got a new partner and despite all the table plans etc being done, and virtually no one having met this new bloke, she insisted he come, or she and her DD, who was a flowergirl, would not be coming.
On the day, she turned up wearing a white, full length lace dress. Not obviously a wedding dress, but inappropriate none-the-less.
The ceremony was in a church attached to a stately home, with the reception in the main house. While all the guests were leaving for the reception, the groom's mother, sister and the new man stayed behind in the church to pose for photos by the alter like an actual bride and groom.
Thankfully, the wedding party were already tucking into the canapes at this point so didn't witness it. It was only when the B&G went to his parents after the wedding that his mum got her photos out (these were pre-digital days) and saw the alter photos when they were flicking through the photos.
Needless to say the new boyfriend didn't last very long after that incident!