I've fallen under the map before, I think about 3 times. One time it happened while running over a bridge in Limsa and I remember it happening twice in East Shroud.

I don't know how or why it happened because I have run over the Limsa bridge thousands of times but that one time it happened. So you can fall under legitimately. They would need to test for other behaviour happening, such as interacting with quests and mobs from below.

It was easy to /teleport out though.