My favourite: "he/she/they is/are a few sandwiches short of a picnic".
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I have a photographic memory and actually memorized a book on idioms after reading it once. Unfortunately, I use non-literal language all the time now and I confuse the other members of the Asperger's meetup group I still have trouble with the types of non-literal language you can't learn from a book.
This pretty perfectly describes my usage of idioms. I read a lot and so have picked up quite a few from my reading which I now use fluently. Non-obvious or novel idioms confuse the hell out me still.