Activating the glitch by saving Mr. Fuji.
Mr. Fuji's house door glitch is a natural glitch in Pokémon Red, Blue, and Yellow, including both the English and Japanese versions.
After rescuing Mr. Fuji from the Pokémon Tower (which occurs before receiving the Poké Flute, normally after getting the Silph Scope from the Rocket Hideout) the player will be placed in Mr. Fuji's house on the right exit mat.
If the player tries to leave immediately by walking down, the door won't work. But if they take one step that is possible (i.e. north, west or east), then the door will work again.