I remember not a wedding, but a 50th birthday party about 5 years ago.
45 people were invited (including me and DH,) and 9 turned up. I felt so bad for the person whose party it was. The people invited were a mix of work colleagues, neighbours, friends, and extended family - cousins, aunts, and uncles and his sister and her husband, (no parents, as they were dead.)
I have no idea why it was such a catastrophic failure, because this guy and his wife were pleasant enough people (we were neighbours.) Of the 12 extended family members - one turned up - a cousin. Of the 13 colleagues, 2 turned up - one with with their partner, (4 people so far.)
Of the 12 neighbours invited 3 turned up - me and DH, and an old chap from down the road. (7 so far.) And then out of the 6 or 7 friends he invited, one came, with his partner. (They made up the 9 people.)
36 out of 45 not turning up - 80% of the guests invited - was humiliating. This guy and his wife and their 2 kids (aged 10 and 11,) moved away a few months after. Completely humiliated, he felt his work colleagues, neighbours, extended family, and friends gave zero shits about him.
And apparently, when they were all invited, (3 months earlier,) every single one of the 45, except 2, said they were definitely coming. 3 or 4 dropped out a few days before; but the rest (still more than 30 people) just didn't turn up.
The ones who didn't turn up didn't even apologise either, OR send a card or gift. Not ONE of them. Can't say I blame him (and his wife and kids) for moving away, when most of the people in his life thought so little of him.
So yeah, people are dicks; nasty dicks. Saying they are coming to your party or wedding, and then not turning up. Terrible etiquette, mean, selfish, thoughtless, and cruel.
I often wonder what this chap and his wife and kids are doing now, where they're living, and if they're OK. I hope they're happy and have people in their lives now, who treat them with more respect and kindness. I have looked for them on social media, but they are not on there (or they are very well hidden.)