The Celadon City origin glitch is a natural glitch in Pokémon FireRed and LeafGreen.
Pokémon obtained with the "CELADON CITY" origin (including Eevee, Game Corner Pokémon and hatched Eggs) will have their origin changed to "CELADON DEPT." upon entering any room of the Celadon Department Store.
The same applies the other way around; Eggs hatched in the Celadon Department Store will have their origin changed from "CELADON DEPT." to "CELADON CITY" upon exiting the department store.
The reason for this is unknown.