The bug

Shields suffer from a sort of gimbal lock: if a player faces straight down, the shield doesn't block any direction. Straight up seems to work ok in my testing, probably due to rounding error of some sort making the facing-vector still have a slight horizontal component.

How to reproduce

Summon a trapped skeleton and stand above and slightly to the side. Arrows are blocked when facing 89.9 down (or at -89.9 almost straight up), but at 90.0 you'll be hit.