I couldn't miss the chance to practice my drawing!
The Duncan bug is a bug in Final Fantasy VI, during the World of Ruin when the player visits Duncan before recruiting Sabin, and before learning Phantom Rush. Whenever the player enters Duncan's home, Duncan will always start off his exercise with two leaps to the right, then go back by leaping twice to the left. If the player enters the submenu after Duncan has jumped to the right at least once, then exits the submenu, Duncan's routine will be reset, thus allowing the player to make Duncan leap to the right as far as they want, resulting in his sprite leaping completely off the screen. There is a bugfix patch that players can use to fix this bug on the SNES version (listed as "Duncan Stays Put" on that page).