And twice that I know of. 
Acquaintance at school has an 8yo DD, who can be a little bit of a challenge.
There's a lot of girls in this year group (23/30) and relationships are quite fluid, but generally ok.
Twice this term, there have been birthday sleepovers involving brthday girl and approx 5 others that X's DD has not been invited to.
So, the solution? Mom phones the party parent to ask "why? and if there's any way to invite the DD as she's really uspet".
Surely, as a parent, you just expalin that numbers are limited, soandso could only invite so many, if there were more, you'd be on the llist, remember how we had to keep numbers tight when it was your birthday?, never mind, friendships change all the time," and on and on until the child is slightly mollified?
AIBU?