Board Thread:Spoilers!/@comment-6175354-20140420035659/@comment-24092059-20140422190752

Dan Fox wrote: It would be interesting if Regina, Snow, and Charming cast the curse, but the price was, that they didn't remember doing it. Glinda might have done it, to prevent Zelena from going back in time and changing things. Similar to Final Fantasy Dimensions. The crystals sensed an evil presence, trying to take them, so they split the world into two. And Zelena happened to have a talisman that allowed her to keep her memories.

That could be it after all zelena never, as far as I know though I could be wrong, has taken off that emerald necklace or broach. Maybe it was Zelena that sent Hook to get Emma and soaked that emerald in the remember who you are potion. =) again speculation and no evidence to back this up.