Abstract: Apparatus for error proofing in manufacturing and kitting operations and the like. A frame having two or four frame members utilizes light emitters and sensors on opposite sides to determine when the plane of the frame has been broken, and storage areas behind the frame have been accessed. This information is compared to programmed information to verify whether errors in accessing the storage area have occurred. The plane of the frame has a plurality of penetration zones distinguishable by their respective corresponding emitter and sensor pairs.