Don't you hate it that if you send out invites (for example a wedding) with an RSVP contact person when you end up having to phone people and they say

"I told your mother/aunt/brother/whoever I was coming. Did they not tell you?"

There's an RSVP contact person's name on the invite for a reason people!

Oh and the next worse. People who see someone else's invite and decide to come too, even when they are not invited. Happened to me when MIL's friends decided to come to my tropical island wedding. Nice.