DIY terrains with gaps in "ground" texture generate glitch maps
From Worms Knowledge Base
DIY terrain is can cause some unexpected and undesirable glitches and errors during the random map generation process. The size, consistently and pattern of the gaps will have various effects on the terrain that range from minor holes and "stalks" to the sort-of vertical interlacing as seen in the screenshot to the right.
So far in testing this glitch has not resulted in any crashes or lock-ups, and these maps may be played on without any problems other than their strange appearance.