The client camera entity is modified by a few mods to be a plain Entity or subclass that doesn't inherit LivingEntity, so this explicit cast throws an exception:

Ah, I'll wrap that in a instanceof check

A more specific example of this case would be the camera entity used by PSWG's ship chase cam.
The question there would be, should that mixin still apply if the player is flying a ship? If so, you'd want a way to either get its "owner" in some way or get the player entity instead.