 Key ghosting occurs when certain combinations of key presses cause additional unintended presses to be registered. You can see how it happens by looking at a keyboard or keypad's internal matrix. A microcontroller powers a matrix column and determines which key is pressed by reading a matrix rows, low, or high voltage state. Ghosting occurs when the controller checks a row and current flows back up through a neighboring closed key switch and out through another closed switch on a different row, resulting in that incorrect ghostly input. But fear not, adding diodes to the matrix can stop current from flowing the wrong way through each switch, and subsequently busting all those ghosts.