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 maps edge is now water.
This glitch can also be observed in Route 5, in both the east and west boundaries.