I suppose my favorite is the "turning of the tables" twist in Bell, Book and Candle where Gillian has cast a spell on a human, Shep, who "falls in love" with her but is clueless as to her magical powers in spite of the way it happened out of the blue (and just before his wedding day).

The twist is she gets "caught" in her own bewitching net (and loses her powers) by falling in love with him - although that's not supposed to be possible with witches.

Of course in fairy tales of various sorts anything's possible.