Poka yoke:

Poka-yoke is a Japanese term that means "fail-safing" or "mistake-proofing". A poka-yoke is any mechanism in a Lean manufacturing process that helps an equipment operator avoid (yokeru) mistakes (poka). Its purpose is to eliminate product defects by preventing, correcting, or drawing attention to human errors as they occur. The concept was formalised, and the term adopted, by Shigeo Shingo as part of the Toyota Production System. It was originally described as baka-yoke, but as this means "fool-proofing" (or "idiot-proofing") the name was changed to the milder poka-yoke.
Examples of poka-yoke in consumer products include:
• Automatic transmissions: the inability to remove a car key from the ignition switch of an automobile if the automatic transmission is not first put in the "Park" position, so that the driver cannot leave the car in an unsafe parking condition where the wheels are not locked against movement.
• 3.5" floppy disks: the top-right corner is shaped in a certain way so that the disk cannot be inserted upside-down.
• UK 13 amp electric plugs: it is impossible to wrongly insert the plug into the socket, due to its arrangement of three rectangular pins.
• Microwave ovens: a door switch automatically disconnects the activation button when the door of the oven is opened. As a result, it is impossible to cook anything in a microwave oven unless the door (which contains a Faraday cage to block microwaves) is fully closed. If it were possible to activate an oven with the door open, this would allow dangerous leakage of high intensity microwave radiation, which would be very harmful to any living creatures in the immediate area.
• SIM (Subscriber Identity Module) cards: The SIM card used in cellular phones has its upper left corner trimmed diagonally to guide the card correctly into position.