well, in egypt ,we say "in the apricot" = "فى المشمش"
that means the thing we are talking about will never happen, I think because apricot rarely or never grows in egypt, so that thing might happen if apricot grows here :)
also we say "when you see the inside of your ear" = "لما تشوف حلمة ودنك" = "عندما ترى حلمة اذنك"
simply because can you see the inside of your ear??? it's impossible!! so the thing might happen if you can do that
also we say "when you bite the back of your neck" = "لما تعض قفاك" = "عندما تعض قفاك"
the same as seeing the inside of your ear :)