[This bit?](https://gist.github.com/indigochill/b96315ee0f7a46fb8f43)

It looks (of course, I could be mistaken) like it would only checks if the player is "inside" the vehicle, or in other words not on an external (outer) part. This would protect from ground-based effects like acid, but not from things falling from above, unless I'm misunderstanding.