I had a thread a couple of years ago but name change regularly so can't remember the one when I posted it BUT ...
a few years ago I was planning DD's birthday. I knew that another classmates birthday was close, so I checked with the mum before we made the arrangements and sent out invitations to make sure they didn't clash. All ok.
36 hours before the party (we hire village hall, do crafts, play games and invite all classmates and friends, so usually about 35 kids, plus my parents fly in to help. There is a lot of effort goes into these parties and the kids love them) she asked me to delay my party by half an hour because 4 of our guests would be leaving her party early, and the birthday package had cost her a lot of money.
So I agreed, that I could delay the big activities until they arrived, Not a big deal. (I did think it was a bit bad form of guests parents, but not my circus).
But she replied, saying actually that wasn't enough time and that I should delay it by minimum an hour. And reiterated that she had paid a lot of money for her party.
Clearly her hired package party for 8 kids was much more important than all of the materials we had bought, food we had cooked, time and effort we had put in.
So when I made it very clear that I was not going to spend my afternoon (the day before the party) contacting many parents to rearrange everything, whilst taking care of my baby and toddler, doing last minute prep and collecting my parents from the airport for their visit, she took a bit of a huff.
We haven't spoken since.