I am using this as a sort of controller with my Arduino (Mega2560) but I can't get real multiple inputs to work. I am able to have say two individual keys pressed at a time but after that they overlap with eachother. (Using the Keypad library also).
Here is an illustration of the issue:
If I press the 3 blue buttons and use the getKeys function it thinks that the red one is also pressed. This works when any three or more buttons are pressed that produce a square.