Confusion and Substitute glitch

The Confusion and Substitute glitch is a natural glitch in Pokémon Red, Blue, and Yellow. If a Pokémon under a Substitute hurts itself due to Confusion, damage will be dealt to the other Pokémon's Substitute, even if it doesn't have one.

The glitch breaking an enemy Zubat's Substitute that doesn't exist. (from Crystal_'s video)

This may cause the game to display a message that the other Pokémon's Substitute broke, drop its sprite and then redraw it.

YouTube video by Crystal_