Today’s invention is an electric lawnmower which can’t cut its own cable.
The cable would be have a number of small sequins embedded in its surface.
A flashgun and camera (red), mounted inside the mower, would fire every half-second or so, powered by the cable itself. This would be set behind a small window which would be wiped continuously by a brush (blue) on the rear face of the blade.
If the camera detected any spots of bright reflection (from the sequins), the power to the blades would be cut off until the cable could be safely moved to one side.