I've not accidentally jumped out of an instance area, but I have accidentally raptor'd off the ley islands at the end of DS, so I'm actually surprised I haven't flung myself out of bounds in any instance yet! I reckon the instance borders should be like ocean borders, just pop you back in if you keep trying to go. I'm also surprised this never happened to me 'cause I have a habit of wandering off. Perhaps they should add a requirement that all players need to be in place before the instance boundaries change?