I was invited to a party (the only invite this year) but it was themed and I wasn't interested in the theme (the theme was anime/manga and you had to dress up and they planned on watching anime/manga too). I didn't really talk to any of the people invited except a couple of them sometimes, and considering I wasn't interested in the theme I didn't go. At first I said I was, and I got my costume sorted, but then I was too scared and made up an excuse of being double-booked.
I only make excuses like that when I know I'm going to be excluded, left out and mostly ignored because it won't make me feel good. But if I'm invited to something, I'll usually force myself to go, because it's so rare I'm invited to anything at all.