Consider the simple scenario: You have two connections on a board that you are trying to wire to another two points. How can you ensure the wires are connected correctly or guarranteed the wires are not crossed? The intuitive simple method I can think of is to have 2 LEDs for each wire, and making sure that the LED light up in sequencial order. Namely, you connect the first wire, the first LED should light up, not the second one. If the 2nd LED lights up, oops, you mis-wired. But this requires trust in the person doing the wiring. What if the person was sleepy that day and overlooked? Is there another way I am missing, that can guarrantted the wires are connected correctly? Any ideas?