with a big thing made out of glass going around it will show unloaded texture (as shown with the 15x15x15 /fill's I guess its because of the way the texture of glass block side are hiden if they are behind the side of an adjacent glass block of the same type but its a bug none the less

and its one that as been annoying me for a little while now and if i recall correctly this bug was not there or less obvious a few update back so its definetly patchable