Randomness and getting burned :-)
Hmmm... having the secret limited to existing words and even reduce it to pets, names and places... the change of brute forcing is on the rise, because it limits considerably the possibilities to check... But as always the balance between security and user experience.
What about facial recognition and finger print revocation or change... Plastic surgery on the rise, or burning finger prints LoL
Once had my finger burned in the kitchen, well yes... I was locked out with fingerprint recognition... Unintended or uncontrolled revocation ;)