This is continuing to hold true, and I've been able to duplicate it. When the "trick" is used, you are able to send mafia invites to people who you are not directly connected to on Facebook. Those mafia invites show up in the ZMC (Zynga Message Center), and when accepted, gives you the option to send a free gift to the person whose mafia invite you just accepted.
As you can see here, it bypasses the usual list of Facebook friends and allows you to send directly to the other person even if they are not a Facebook friend.
When that free gift request is sent to the non-friend, it shows up in their own ZMC, and also on their Facebook requests page.
If the gift is accepted, it gives an error, both through the ZMC and if the request is accepted from the Facebook requests page.
So I guess now we know that error code af-77 is due to not being Facebook friends with the person who sent you the gift.
