I've had some sort of party problem issue every year for DS.
One party he was only allowed to have 5 from the class and my good friends DS was in the same class but not a good friend of DS so wasn't one of the 5. I knew she would be miffed at not being included so I said it gently and invited her DS separately to a little gathering at our house but you could tell she was pissed off for a while.
Same gathering, my best friend said yes for her 2, then one of the 2 got a better offer was invited to another so she spoke to me about it and accepted the better offer second invite. I told her it was ok because what else can I say? "no your child has to be at my party" which she would have probably been pissy about.
Same thing happened last year with another friend. She accepted for both of her children. Then her DS got an invite to another party and again she spoke to me about it. Her DS even told her that he should stick to the 1st party but it was his mum who said she would speak to me. Nice to know the child had more etiquette than his mother. Again, how was I suppose to say know without her getting pissy.
One year my entertainer accidentally cancelled my booking for whatever reason it was. I was sat with 20 minutes to go in an empty room and had to go and break to news to DD who was waiting at home.
Year before last a friend had booked her childs party that overlapped mine (not sure how it happened tbh as her DC were invited and before she had done it) and a lot of the guests were mutual friends who had already accepted. She asked me if I could change mine as the entertainer she had only did that time slot. No it was not the day of her childs birthday but it was of mine.
Last year DS's invites went out at school, his best friends mum came to me in a panic as her DS's party was the hour after mine had finished and she was worried no one would go to hers. We did but that was some rush!
This year it looks as if the one person DS really wants to come isn't as he may be going to another party but I haven't heard either way and DS isn't overly happy.
There should be rules! Always stick to the first acceptance because anything else is rude no matter how you say it, get invites out a decent amount of time so if there is a clash, it potentially can be sorted out. And adult friends are not allowed to sulk if their child doesn't get invited to a child's party that they don't even play with!
Needless to say its rare that any party goes without a hitch, there always seems to be something. Second mother should have been more organised and now she does know then she should change it seeing as the invites haven't actually gone out yet.