When I was ten, there was a new kid in my class. We shared a bus stop, and I thought we were making pretty good friends. His birthday party was the talk of the school - everyone was looking forward to it, talking about it, planning for it. I didn't receive an invitation. It had been nearly two years since I had been invited to a birthday party for a classmate, and I was devastated.
My mother called his parents, told me it was some kind of mistake, and I went over. I had a good time, and yet...I know it's possible I wasn't supposed to be invited. I know it's possible I wasn't invited and my mother made them invite me. And that bothered me. It still bothers me. To this day, I'd rather sit at home doing nothing than risk being somewhere I'm not wanted.
I found out after-the-fact that a friend's wedding was "open invite". I missed out on a truly stupendous amount of prime rib. But "open invite" means different things to different people, and I'm not coming unless I'm sure you want me there. I don't need an engraved invitation, I just need to know that you know that I might be there.