An interesting example of Folk Etymology is curry favour meaning to ingratiate oneself by flattery. First, curry is the word meaning to brush or groom a horse, not the Indian spice. And the second word was originally Fauvel, the name of a horse in a satirical French medieval poem from the early 1300s. The French word fauvel from which the horse was named meant "fallow" or "chestnut-coloured" among a few other meanings, but was not related to favour. Fauvel, the equine hero of the poem, was a cunning rascal, the sort of character that might be influenced by flattery or favours.
The phrase to curry Fauvel developed meaning to ingratiate yourself with someone hoping for a favor in return, like you would by currying the rascally horse. At the time that the phrase developed in English, the poem was well known to educated people in Britain. As time went by the poem, and therefore the original meaning of Fauvel, was forgotten, and the word favour was substituted. While the word favour made perfect sense with the meaning of the phrase, the verb curry remained as a curious puzzle.
By the way, the word for the spice curry came a few centuries later from Tamil via Portuguese.
Source: POSH and other language myths by Michael Quinion, 2004, Penguin Books, London