Texture offset anomaly
The way the column renderer in Doom works allows all textures to be larger than their actual dimensions as defined in TEXTUREx. However Dday only accounts for this with sky textures.
An example of this can be found in this HeXen pwad: http://www.doomworld.com/idgames/index.php?id=6291
Sectors 32 and 33 (and numerous other's) feature a custom mid texture on their back walls that replaces the BLANK texture in the original HeXen. However the patch used to make said texture has an odd offset that the original games column renderer could deal with meaning the texture is displayed correctly in HeXen. Dday however doesn't display this texture correctly.