The 2x2 block encounter glitches are a group of glitches in Pokémon Red and Blue which were fixed in Pokémon Yellow.
These glitches occur when the game checks an encounter for a 2x2 tile, such as the left-facing shore tiles on Sea Route 20, or the grass tiles with a star on their bottom-right corner in Viridian Forest.
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 'shore' (left side of the tile) counting as grass encounters and 'water' (right side of the tile) counting as water list, left-facing shore tiles can bring up unintended grass encounters. Similarly, 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.
- Grass/rock Surfing glitch (Spanish, Italian versions only) — These glitches fixed the left-facing shore tile glitch but seem to have allowed water encounters with a grass rate encounter rate if the player is Surfing on grass or a rock via the Surf down glitch. The water list can be manipulated by fighting a Trainer beforehand.
- English grass/rock Surfing glitch — The water encounter rate (manipulated after encountering a Trainer) can be forced to remain as a non-00 value through a trick that involves use of the glitch items 9F (hex:5E) and 8 8 (hex:7C).