In the map abuse, there are some square holes on the main bridge (the large, partly diagonal one). A shadow is cast on them (valpha 0.001 surface, as you can see the below is still using the default 0.5 value), projectiles and players don't go through them. It's confusing, because you would think you would fall and thus die. A proper transparent surface should be put.
The main bridge also looks quite bland.
If this is a confirmed bug, move to the issue tracker.