Super Mario 3: Mario Forever: Secret

At the very beginning (when you choose which save to play), if you go to the right, you can choose a bonus – a mushroom (costs one life) or a flower (costs two lives). So – if you jump to the rightmost step (in front of which there is a flower), and then jump to the maximum to the right and up, you will find yourself on a secret level. (tested on version 2.16).