Olivine City has a hidden map/house that is only accessible by using cheat codes.
The map is of a small house containing two NPCs. The first is a Rhydon, whose glitched sprite changes into a Clefairy during walking animations. When A is pressed in front of it, it simply says, "Gugooh!" The next is a woman, when A is pressed in front of the woman in the house, she says: "When my Pokémon got sick, the Pharmacist in Ecruteak made some medicine for me." It is worth noting that she says that the pharmacy is located in Ecruteak City, when in the final version of the game it was located in Cianwood City.

Also, presuming that the player is in the Olivine House; if the radio is playing "The Pokédex Show" instead of the "Pokédex Talk Show" the game will not display any Pokédex entries and freeze (this will occur between 4am and 10am).