Sprites extend by one pixel width vertically
From Worms Knowledge Base
In some versions of Amiga WinUAE, the sprites for worms, projectiles and in-motion gravestones will be rendered twice on top of each other, resulting in an effect that appears to extend the pixel width of the sprite once vertically to the right.
In worms this creates a sort of "fat worm" appearance. With in-motion graves it makes them appear slightly chunkier, although this disappears once the grave has settled and becomes destructible terrain.
This bug appears to be fixed as of WinUAE version 2.3.1.