Ouch.

code/modules/mob/living/living_movement.dm

@@ -2,6 +2,21 @@
 	. = ..()
 	update_turf_movespeed(loc)
 
+/mob/living/CanPass(atom/movable/mover, turf/target)
+	if((mover.pass_flags & PASSMOB))
+		return TRUE
+	if(istype(mover, /obj/item/projectile))
+		var/obj/item/projectile/P = mover
+		return !P.can_hit_target(src, P.permutated, src == P.original, TRUE)
+	if(mover.throwing)
+		return (!density || lying)
+	if(buckled == mover)
+		return TRUE
+	if(ismob(mover))
+		if (mover in buckled_mobs)
+			return TRUE
+	return (!mover.density || !density || lying || (mover.throwing && mover.throwing.thrower == src && !ismob(mover)))
+
 /mob/living/toggle_move_intent()
 	. = ..()
 	update_move_intent_slowdown()