+ I had a NaN position while experimenting with (directional) gravity
+ blocks recently. I checked the gravity code and it seems solid, so
+ might be something I overlooked or, what more probable, it triggered
+ some NaN condition in the collision code. Needs investigating
+
+ This may be related to the HUD locking on an entity. If the entity is
+ teleported to a NaN position, the ray intersection test could give a
+ false positive.
+