I believe I have found a bug with the moat diggers on finite worlds. Having just made myself a ribbon world map I've found the moat digger to only work in the infinite direction, in the limited map size direction it complains either that the thing is undiggable, or that an obstacle has been detected and it is shutting down. (Factorio 0.14.21 and Terraforming 0.14.2 200 high n/s infinite e/w)
