D's parents send out Christmas cards to all of their guests every year, so they have addresses. What they don't have are names. They have one couple that is unmarried, and they send invitations to His Full Name & Hershortenedfirstname... I don't even think they know what her last name is. The problem came up more than once when I was helping D address his graduation announcements last summer! And I kind of want to do this right, you know?
And don't even get me started on postage. Many of my dad's guests are in Pakistan, and I don't even want to think about how much it will cost when I actually mail them invitations! (And what kind of postage do you put on the RSVP postcards? My god!)
Oh, and of course there is the problem presented by my grad student friends. Most of us move away for summer internships (or fall internships), so even if I get their addresses this weekend for the save the dates (which I should already have because of how often I am at their apartments, but I am never paying attention to what street I'm actually on), I'll have to get them again for the invitations anyway. Annoying!