Board Thread:General Discussion/@comment-6036560-20130318104316/@comment-4260948-20130425023106

Yes, it's only a theory, but it's been backed up by evidence. We have two people - Belle, Dwarf - who have lost their memories crossing it and two people - Greg, Kathryn - who have had accidents crossing it.

Thus, while not confirmed, it makes sense that Kurt would have lost his memories - since he was inside Storybrooke forest when the curse was cast - when he crossed the line attempting to follow Owen. See?