When game developers test, implement and remove parts of a game's functionality, it's common for parts of the code to still remain inside the game, even though the leftovers are usually harmless. In the early stages of Street Fighter II, a 'dizzy damage' feature was initially planned, meaning a stunned opponent would take double damage if they were struck whilst in their dizzy animation. Due to part of the code that wasn't removed, there's an exploit in the game that causes Ryu to take double damage in his stun sprite, but only in one particular frame.