had the same problem recently.
my sim threw a birthday party, invited a bunch of others, including his pregnant wife who didn't live with him. the party started fine, until about 20 sim mins in, where it told me it had failed and it ended. there was no reason for the failure or the early ending as far as I could see. everybody left, but the next day my sim's wife turned up on the lot again (she has a key to the apartment), and the birthday party popped up again, and has done every time she turns up since. no other sims turned up though.
I'm guessing that the failure was due to her going into labour while the party was going (even though my sim didn't get the panicky icon), and then it kind of got stuck on her. the thing is though, every party from that point on got stuck on her too. even though the other parties completed, and went fine, every time she turned up the last kind of party would pop up.
unfortunately, the sims in question are now ghosts, and the odd party spamming doesn't happen anymore.
Paying closer attention I've noticed something: The dinner party pops up when friends with keys "pop in" with the normal "Your day just got 100% better" sort of visit, and then the timer for whatever the last hosted event starts again, but in a borked way.
The only way I know to reset it is to save and back out to the main menu. When I have my next party I'll back out right after, and see if that stops it from happening.
