In a sheep farm (the building not the fields) the Centre area (within the wall) is blank leaving an area of screen which is not updated, keeping what was last on those pixels when the screen in scrolled or the mouse pointer is moved over it.
I cannot replicate this (the issue with the fields is well-known however, and a problem with the exe). Which version are you using? Any screenshots?
Imho, the parameter needs_ground is missing in the dat (in all definitions of the sheep farm).
Thanks for spotting this - that should fix it. I will upload a fix to SVN later (but it's too small to do a new binary release of the whole pak at this stage I think).