I have had this happen a number of times. I guess the sim decided he didn't want a party after all or he is just a rude sim, lol. Anyway, not sure why it happens. Sometimes the balloon icon is there and by the time I get my sim out of the house, the icon is gone. I think one time, even though my sim didn't go to the party because no one was home, he still got the message, "thanks for coming to my party". Silly sims.