The problem happens when you press three or more keys at once in right combination. And is exactly the same regardless whether you scan output wires in sequence (as is done in simple hardware based keyboards of the 70's and 80's) or if you read them all at once.

Going back to your original example, if one would press keys 1, 2 and 6 at the same time your microcontroller would falsely detect 7 as being also pressed (because keys 1 and 6 shorts two input wires together). This assuming that your output register drives strong 1 and weak (preferably floating) 0. If it has strong drive for both 0 and 1, then such combination of pressed keys would cause some unpredictable effect, possibly including burning out the pins.