In the first screenshot, taken in E2L2, you can see that the sentry turret is stuck halfway inside the wall while dormant, and is unhittable. In DOS Duke 3D it is always outside of this wall and can be shot.
When activated, it may either be launched out into the air, or into the ceiling of the other sector. In the latter case, it becomes impossible to destroy the turret. The following screenshot shows the former case:
Even without the turret being lodged inside the wall, they can still move around seemingly on their own accord (E3L2):
Testing indicates that the cause appears to be revision r8001:
Quote
Call A_GetZLimits() for sprites in the map at load time