I can beat all these - BIL (DH's DB) marrying my now SIL - invites arrived, family children only invited. Fine, RSVPed we'd be bringing DS - discussed in person with them getting him a little outfit (he was 18months). They were marrying around Easter, when my parents would normally go on holiday, so in January I got DH to check once more with BIL that DS would be wanted because if not, my parents could have DS overnight, but they'd need to make sure they were back for it. Told again, they definately wanted DS there, but first comment that not in the evening do. I said not a problem, we could sort that.
5 weeks before the wedding (and 8 months after first told that DS would be welcome), I got an e-mail from SIL saying they'd thought about it and decided which children might ruin the day, and DS was no longer welcome at the ceremony or the dinner. (On her side, she did still have the DCs from one of her brothers but not the children of the other side, must have been fun in their family sorting that out).
It then transpired that while DS wasn't wecome at any of the key events, (ceremony, meal, evening do) it was expected we'd bring him in a nice little outfit for photos and the drinks reception. Which meant hiring a nanny for the day/evening - costing around £200.
I was tempted to say sod it, I'd stay at home with DS, DH could go to the wedding on his own, but MIL got a bit upset (there were a lot of other bridezilla moments and it was all getting a bit much for PIL) so for the sake of family harmony, we decided to swallow the £200 costs of a nanny and hte general insult that we'd like DS ruin it (I'd even previously said I'd sit at the back with him and walk out if he made any noise).
Thing is, DH has never forgiven SIL, it still causes problems to this day. DH and BIL used to be close and now they aren't. It's a real pity that some people put having their 'perfect day' above acting with consideration to others. I'm sure if we'd had a 'no children' invite from day one, it would have caused so little upset in comparassion, generally it's the lack of thought that creates the ill feeling.