The Viridian Forest no encounters grass tiles glitch is a 2x2 block encounter glitch which occurs in English Pokémon Red and Blue and was fixed in Pokémon Yellow.
Encounters are disabled for the grass tiles with a 'star' for their bottom-right sub tile. Due to this, players can avoid encounters by walking through the forest in a certain way; which is useful for speedrunning.
Image explaining how the encounters on the 2x2 block work.
In English versions, the left side of the tile controls the encounter list source (grass, water or none) and the right side of the tile controls the encounter rate source (grass, water or no encounters). Due to 'grass' (left side of the tile) counting as grass encounters and 'star grass' (right side of the tile) counting as no encounters, encounters can be avoided in Viridian Forest by walking on the star grass tiles.