Dupli's get stuck at end of cycle

upon completion of a full bar on a tile, (dig, construct, build, etc) the task does not end, leaving the Dupli forever digging nothing, or the equivalent. Can be cancelled but always results in an invisible tile that cannot be built upon sometimes this creates a physical obstruction but other times the tile simply remains "blank" - ie: you can walk through it and objects fall through the space but nothing can be placed/built on tile