#45 - Castaway by Exchange

In Route 226 from Pokémon D/P/Pt, there's an island where a Finneon can be traded for a Magikarp. However, if the player doesn't have any Pokémon that can learn Surf, Fly or Teleport, the character will remain stranded in that island forever as a result.

SOURCE: https://bulbapedia.bulbagarden.net/wiki/Sinnoh_Route_226#Trivia