In opengl mode only, if you put a midtexture on a sector with a sky ceiling, and set the midtexture to scroll downward, it will scroll out of view. I tested this on today's dev build of gzdoom. It also happens in 3.2.1, and GZDoom 1.8.06 (Zandronum 3.0).
Here's what it looks like in software:
In older versions of the opengl renderer (e.g., gzdoom r900 / Zandronum 2.1.2), the texture will not scroll out of view,
See attached wad for an example.