Don't order any of the faerie food,"said Jace, looking at her over the top of his menu. "It tends to make humans a little crazy. One minute you're munching a faerie plum, the next minute you're running naked down Madison Avenue with antlers on your head. Not,"he added hastily, "that this has ever happened to me.

Jace is warning the person he is with not to get any food from the faeries. He is saying that the food can make humans act strangely and lose control. He is giving an example of the kind of dramatic effect the food could have, saying that someone might start running down the street without any clothes on with antlers on their head. He is quickly adding, though, that this has never happened to him.