Here's the quirk: male friends who are in a partnership with a female for whom I have a folder get filed under her folder. This applies to ex-boyfriends, too, and, most amusingly, resulted in a friend's wedding announcement (the wedding is to someone else) ending up in the folder of mail I have under his ex-gf's name. This is completely intuitive to me, which is why it works, but it has confused at least one person immensely. :)
What's a little quirk of yours?