This glitch involves a flaw with the map borders.
- Go to Cycling Road though the Celadon City entrance.
- Bike to the left until the player is at the edge of the map. Left of the maps edge, you will see trees.
- Once there, bike downwards until water is on the left of the maps edge.
- Turn around and bike upwards again.
- Back up Cycling Road, some of the map's edge is now water.
This glitch can also be observed in Route 5, in both the east and west boundaries.
Each map has a background tile, which is used to fill the area outside the map boundary. This glitch happens when connecting maps have different background tiles.
When the player enters a map, either through a warp or through a map connection, a "tile block map" is built for the entire map. The tile block map is larger than the actual map by 6 steps in all four directions, so that it always covers the entire screen no matter where the player is on the map. Initially the tile block map is filled with the background tile, then data from the current map is loaded in the middle, and data from connecting maps are loaded on the corresponding borders. Normally this ensures that the tile block maps of connecting maps will always agree; however, tiles outside the boundaries of both maps are left as the background tile for the current map, which may be different for connecting maps.
This is the case for the north part of Cycling Road: The background tile of Route 16 (Cycling Road entrance) is trees, and the background tile of Route 17 (the sloped part) is water. When crossing the map boundary, the map doesn't suddenly change, because the tilemap in the VRAM is only updated as needed. But as the player takes steps, part of the background will scroll onto the screen, and whether trees or water is loaded depends on which map the player begins the step on.
Opening a text box (e.g. the Start menu) switches to another tilemap in the VRAM which is updated every frame, allowing the player to see the actual background tiles for the current map. Notice that the tilemap is still only updated at the beginning of each step or after a text box is closed, so if the player opens the Start menu immediately after crossing the map boundary, the background tiles for the previous map is displayed instead; only after taking one more step or opening the Start menu the second time will the background tiles correspond to the current map.