Basically, under some rare circumstances, the place where the buildable tests are carried out is not the place where the buildable is... placed.
Yes, but this is not that case. All turrets are exactly there where they are :-)
edit: what you'd have to do to allow this in normal play I'm not sure. Probably it'd involve changing tracemasks or the r.contents of some buildables so they appeared to the server similar to the structural brushes of the map.
I think that is easy. But I am not C coder, so it's hard for me. Must be changed only one thing - allow build turret into air.
Now is detected place for buildable on map, if it's free place (square place with buildable dimensions), you can build.
If is other buildable on that place, you can't build.
But, it should be changed to: If is other buildable on that place, get what buildable it is, get their height and check free place on map floor coordinates + height blocking buildable (above that building). If it's free place, you can build there (practicaly in air).
That's all. But I don't know where and how add height number to checking place for buildable :-(
Sorry for my bad english.