If the failed LED becomes an open circuit, the voltage between the LED's 2 electrodes may build up until it finds a path through two other LEDs. There are as many such paths as there are pins used to control the array minus 2; if the LED with anode at node "m" and cathode at node "n" fails in this way, it may be that every single pair of LEDs in which one's anode is node "m", cathode is "p" for any value of "p" (with the exceptions that "p" cannot be "m" or "n", so there are as many possible choices for "p" as the number of pins controlling the array minus 2), along with the LED whose anode is "p" and cathode is "n", will all light up.