Yeah see I am not nice person like that. If they don't rsvp I just cater for those that have. If food falls short "oh well sorry did you reply to say you were coming? No? Well since you didn't I assumed you weren't"

Annoys the hell out of me.
I have to say, I have done this. I also find it the height of rudeness, that people can just expect to show up without bothering to rsvp. It takes all of thirty seconds! I have literally just said "Well you're here now so you're welcome to stay, but I cannot guarentee that there will be enough to go around for you. My priority is getting the people who rsvp'ed fed, and if there's anything left over you might be able to have some. Sorry!"