Witchy Fiction discussion

"Mud and silk, blood and milk, never the twain should meet. For if they do, bad luck to you, Tis the Devil you'll greet." (Smith, 8)

I think rhymes and warnings of this sort, especially the cryptic ones, are so interesting. From a very superficial search, I don't see that this particular rhyme has roots outside of Smith's imagination, but I don't know.

In any case, are there any warnings or rhymes that you've known or lived with that you particularly love? Any you find to be particularly true, interesting, or witchy?