The user using Substitute on ¼ of its HP.
The Substitute ¼ HP glitch is a natural glitch in Pokémon Red, Blue, Yellow.
If a Pokémon uses Substitute on exactly 1/4 of its HP, it will faint as soon as the Substitute appears, instead of the player receiving the "Too weak to make a SUBSTITUTE!" message. This behaviour was changed in Pokémon Stadium so that a Pokémon can't use Substitute on exactly 1/4 of its HP.
- ChickasaurusGL (article text, video)