I noticed in a map that I was playing that enemy ammo drops were ending up floating off the ground if the drop happened near what I thought was an impassible 2S mid texture. However, on examining the level, the collision for the 2S line was actually handled by using bridge things.
Load up the attached WAD and type "kill monsters" at the console (I use Strife style ammo drops - which may be necessary). The enemies will die and spawn their clips, some of which will come to rest at the height where they met the bridge objects.
Here is a screen shot of the test level. All the ammo clips in the image have stopped moving:
Around the border of the red area are a bunch of bridge objects (type 9990) set to have a "radius" of 8 and a thickness of 128. ie the bridges are the full height of the room so the clips are sitting in, rather than on, the bridges.
I'm not really sure what the correct behaviour in this situation should be. Perhaps the current behaviour is correct. I just thought that it was worth flagging up.