The Pokémon Tower Pokédex glitch is a glitch in Pokémon Red, Blue, and Yellow.
In Lavender Town's Pokémon Tower, seeing a Gastly, Haunter or Cubone disguised as a Ghost will register it as seen in the Pokédex, even if the player has not revealed its identity with the Silph Scope.
Normally, the effects of this glitch cannot be seen with Cubone, because there is an unavoidable Pokémaniac with a Cubone in the first area of Rock Tunnel from Route 10.