Not sure if this is a mitigating factor, but (according to Carrie) the host of the other party had been trying to arrange it for six months, but various members of this group couldn't make this or that date. The only date that suited everyone (except Carrie) was Carrie's birthday - and the host chose this at the last minute. Apparently this party was kind of a Big Deal with this group, and everyone had been talking about it for ages, and Carrie really wanted to go...I think it is, but I'm pretty surprised that if this party and getting together with the other attendees was so important to Carrie, that she hadn't invited any of them to her own birthday celebration, which would have taken the date out of her calculations?
That's a good point. So Carrie invited 5 people to her birthday party, and none of those 5 were part of the Big Deal group? My assumption would be that the 5 people invited to her birthday party are her closest friends, therefore no one in the Big Deal group are among her closest friends. And, if anyone from the Big Deal group had
been among her closest friends and thus invited to her birthday party, they would be going, "Oh, this date doesn't work either, we're already committed to something else. We'll have to have the Big Deal party on a different day."
Honestly, with good friends, I could still see something being worked out, because it really doesn't matter if objectively something is rude, if everyone involved is okay with it. But, it sounds like with Carrie, this isn't a one-time, weird occurrence that she realizes is an imposition--it sounds like with her, asking others to inconvenience themselves to accommodate her is more like Plan B in her mind, a perfectly reasonable option. And yet, it's not like she's just a free-wheeling, spontaneous person who is okay with plans changing on the fly--when someone else wants her
to change plans, even by just a little bit (12:45 vs. 12:30 for lunch), she makes it very difficult for them.
People aren't perfect and you can certainly have give and take in a friendship--that's what friends are, people you feel close enough to, to give them leeway you wouldn't give general society. But that leeway has to go in both directions, and the pattern as described shows that Carrie is always the one taking and rarely the one giving.