The diploma curiosity occurs in Pokémon Red, Blue, and Yellow.
A Pokédex with 150 entries will count as completed according to the Game Freak staff in Celadon Mansion even if one or more of the first 150 Pokémon is not registered as owned (due to Mew No. 151 and/or or Pokémon No. 152 being registered in the Pokédex). This allows the player to receive the diploma.