We got about 2/3 of our RSVPs without any uninvited write-ins, but I just got one where the [invited] couple wrote in their [uninvited] baby on the RSVP. I'm irritated that now I have to send them the awkward "we're sorry we can't accommodate [speshul snowflake]; we hope you can still make it."
I'm worried that if they decide to come anyway they will be smiling and nice on the outside but just thinking "OMG YOU EVIL BABY-HATING WITCH" the whole time on the inside.