The whole feed in in the enclosure= accidental prey bite cannot happen unless you reach in exclusively to feed the snake. This is classic conditioning. For classic conditioning to work it has to happen every single time in a consistent manner. If you open a tub or terrarium only to feed your snake and no other time then yes, the snake will be conditioned. However most responsible breeders and pet owners inspect tubs and terrariums almost daily. Because of the inconsistency, the classic conditioning will not work. This is for ALL animals, not just exclusive to snakes. So I wouldn't call it a myth because it can happen but under strange conditions. Most zoos will feed large snakes in a separate tub or area so they can do cleaning around the living quarter without the animal in the enclosure. I don't know how frequently the zookeepers visit the animals, but if its that infrequent then I would not be surprised if they fed in a separate area.