Confusion and Substitute glitch

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

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.

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_