During tax season (in US), for security, you may optionally create a 5 digit pin. I wanted mine to be unaffiliated with any existing pin I have, and chose a 'random' number and wrote it down. A year later I repeated this (and had long since forgotten the previous pin), went to go write it in the same place, and saw that both 'random' numbers had the same first four digits. I now use a computer to choose random numbers because I no longer trust myself to be random.