User blog comment:Mrob27/Trial (and Error) by Turrets/@comment-4501235-20111217061528

There are a few other odd things I've noticed about turrets:
 * You can't place a turrent on a block that has any Entity hovering above it.
 * You can place a turret on top of any block that you can 'reach' as long there is nothing directly on top of that block. This is covenient if you are in a trench or a hole and there are Astronauts above you trying to kill you.  In fact this is one of my favorite methods for dealing with astronauts; quickly dig myself into a 9 block deep hole and then place turrets at the top until the badies are all dead.
 * Turrets are the only placeable object that won't affect the Transparency Bug. Usually if you're at a transparency boundary and you destroy or place a block (even a Mini Light) beyond the transparent faces of the blocks they will start rendering properly again.  But not in the case of turrets, I think is more evidence that turrets are more like Mobs than Devices.  You were right after all.