Cliff glitch

From WikiBound The cliff glitch is a glitch in EarthBound where it is possible for the player to clip through certian parts of a cliff, and skip parts of the game.

Cause
EarthBound's maps are divided into tiles. The game's programming designates each tile as either passable or impassable, with all cliffs being impassible. Because of the jagged shape of cliffs, they sometimes form 1-tile wide "nooks" where 3 of the 4 adjacent tiles are impassible. If this nook is diagonally adjacent to a passable tile on the other side of the cliff, it's possible for Ness to squeeze into this nook, and then cross over into the diagonal tile.

Executing the glitch
A cliff cannot be crossed by walking diagonally alone; it requires a combination of horizontal and vertical movements. The best way is to move in various directions, such as right, left, and down to squeeze through the nook. Failure can occur if you walk too far to the left before turning down because this can cause Ness to get pushed out of the nook or off the cliff. The nook itself can be entered with diagonal movement, but it requires precise positioning. Also doing this glitch with a Skip Sandwich can make things easier.

Uses
While using the walk through walls glitch allows the player to go anywhere on the entire map, player cannot interact with anything, and requires entering a door, cave, or touching a staircase to get back to normal state. This glitch is a faster way to go out of bounds and allows the player to still be able to interact with things. This glitch is commonly used by speedrunners to bypass parts of the game as it is faster.

Skipping Twoson
One of the most well-known uses of the cliff glitch is skipping Twoson. The set of cliffs south of the bus station can be crossed to access a road segment that connects Threed and Dusty Dunes Desert (see the game map for reference). From here, entering the haunted tunnel leading to Threed will cause the ghosts to carry Ness into Threed because this tunnel is normally intended to block progress into Dusty Dunes Desert until Master Belch is defeated. While skipping Twoson also skips rescuing Paula in Happy Happy Village, the game adds Paula to your party when Jeff reaches Threed.

Check Area Glitch in Onett
This glitch can also be used to access the northeast corner in Onett where the player can preform the check area glitch. This is one example of the benefits of using this glitch as opposed to the walk through walls glitch as the player can still check the corner to preform the glitch.